The study investigates the relationship between chemical oxygen demand (COD) and total organic carbon (TOC) for a wide range of organic compounds, including laboratory chemicals, pharmaceuticals, and pesticides. The COD measurement is known for its long reaction time, expensive and toxic reagents, and the use of potassium dichromate, which is carcinogenic and mutagenic. In recent years, TOC has become a more popular alternative due to its faster analysis time and independence from the oxidation state of organic matter. The study found that the experimental TOC values were generally close to the theoretically calculated ones, with a difference of less than ±4%. The COD/TOC ratios for most compounds were between 2.0 and 3.0, with higher ratios for macrolides and lower ratios for fluoroquinolones and tetracyclines. The structural dependence of these ratios suggests that a single multiplication factor for TOC to approximate COD is not suitable and should be determined individually. In advanced oxidation processes (AOPs), the COD/TOC ratio changes during degradation, indicating that TOC cannot be a reliable substitute for COD in such studies. The authors conclude that while TOC can replace COD in some cases, it should be regularly checked to ensure the ratio remains stable.The study investigates the relationship between chemical oxygen demand (COD) and total organic carbon (TOC) for a wide range of organic compounds, including laboratory chemicals, pharmaceuticals, and pesticides. The COD measurement is known for its long reaction time, expensive and toxic reagents, and the use of potassium dichromate, which is carcinogenic and mutagenic. In recent years, TOC has become a more popular alternative due to its faster analysis time and independence from the oxidation state of organic matter. The study found that the experimental TOC values were generally close to the theoretically calculated ones, with a difference of less than ±4%. The COD/TOC ratios for most compounds were between 2.0 and 3.0, with higher ratios for macrolides and lower ratios for fluoroquinolones and tetracyclines. The structural dependence of these ratios suggests that a single multiplication factor for TOC to approximate COD is not suitable and should be determined individually. In advanced oxidation processes (AOPs), the COD/TOC ratio changes during degradation, indicating that TOC cannot be a reliable substitute for COD in such studies. The authors conclude that while TOC can replace COD in some cases, it should be regularly checked to ensure the ratio remains stable.