This study investigates the antioxidant capacity of 169 phenolic compounds using the DPPH assay and analyzes their structure-activity relationships (SAR). The DPPH assay is a widely used method for evaluating antioxidant capacity due to its simplicity, cost-effectiveness, and speed. However, the results can be influenced by experimental conditions such as reaction time and concentration, and there is no standardized protocol. This study aims to establish the DPPH assay as a reliable tool for screening antioxidants by analyzing a large dataset of phenolic compounds under uniform conditions. The study measured the DPPH radical scavenging activity of 169 phenolic compounds and calculated their Trolox equivalent antioxidant capacity (TEAC). The TEAC was determined based on the 50% inhibitory concentration (IC50) of DPPH radical scavenging. The results showed that tannic acid had the highest TEAC of 15.4 TEmol/mol, while many commercially available simple phenols had TEAC values ranging from 0 to 4.0 TEmol/mol. The study also compared the TEAC values with those obtained from the hydrophilic oxygen radical absorbance capacity (H-ORAC) method and found differences in the results due to variations in measurement methods and mechanisms. The study identified that the ionization potential (IP) was the most important factor influencing the antioxidant activity in the DPPH assay. Compounds with higher IP values generally showed lower antioxidant activity. The study also found that the number and position of aromatic hydroxy groups significantly affected the TEAC values. For example, compounds with two or more hydroxy groups in the same orientation exhibited higher TEAC values than those with only one hydroxy group. The study also compared the TEAC values of phenolic compounds with those from the FooDB database, which contains tens of thousands of compounds. The results showed that many phenolic compounds in FooDB had TEAC values greater than 0.2 [molTE/mol], indicating their potential as antioxidants. The study also found that the TEAC values of compounds with electron-donating groups were generally higher than those with electron-withdrawing groups. Overall, this study provides valuable insights into the structure-activity relationships of phenolic compounds and their antioxidant capacities. The results suggest that the DPPH assay is a reliable method for evaluating antioxidant capacity, and the findings can be used to develop new benchmarks and large-scale in silico tools for molecular design. The study also highlights the importance of considering factors such as IP, electron-donating and electron-withdrawing groups, and the number and position of aromatic hydroxy groups when predicting the antioxidant activity of phenolic compounds.This study investigates the antioxidant capacity of 169 phenolic compounds using the DPPH assay and analyzes their structure-activity relationships (SAR). The DPPH assay is a widely used method for evaluating antioxidant capacity due to its simplicity, cost-effectiveness, and speed. However, the results can be influenced by experimental conditions such as reaction time and concentration, and there is no standardized protocol. This study aims to establish the DPPH assay as a reliable tool for screening antioxidants by analyzing a large dataset of phenolic compounds under uniform conditions. The study measured the DPPH radical scavenging activity of 169 phenolic compounds and calculated their Trolox equivalent antioxidant capacity (TEAC). The TEAC was determined based on the 50% inhibitory concentration (IC50) of DPPH radical scavenging. The results showed that tannic acid had the highest TEAC of 15.4 TEmol/mol, while many commercially available simple phenols had TEAC values ranging from 0 to 4.0 TEmol/mol. The study also compared the TEAC values with those obtained from the hydrophilic oxygen radical absorbance capacity (H-ORAC) method and found differences in the results due to variations in measurement methods and mechanisms. The study identified that the ionization potential (IP) was the most important factor influencing the antioxidant activity in the DPPH assay. Compounds with higher IP values generally showed lower antioxidant activity. The study also found that the number and position of aromatic hydroxy groups significantly affected the TEAC values. For example, compounds with two or more hydroxy groups in the same orientation exhibited higher TEAC values than those with only one hydroxy group. The study also compared the TEAC values of phenolic compounds with those from the FooDB database, which contains tens of thousands of compounds. The results showed that many phenolic compounds in FooDB had TEAC values greater than 0.2 [molTE/mol], indicating their potential as antioxidants. The study also found that the TEAC values of compounds with electron-donating groups were generally higher than those with electron-withdrawing groups. Overall, this study provides valuable insights into the structure-activity relationships of phenolic compounds and their antioxidant capacities. The results suggest that the DPPH assay is a reliable method for evaluating antioxidant capacity, and the findings can be used to develop new benchmarks and large-scale in silico tools for molecular design. The study also highlights the importance of considering factors such as IP, electron-donating and electron-withdrawing groups, and the number and position of aromatic hydroxy groups when predicting the antioxidant activity of phenolic compounds.