This study investigates the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) kinetics on platinum (Pt) in both acid and alkaline electrolytes. The research focuses on polycrystalline platinum (Pt(pc)) and high surface area carbon-supported platinum nanoparticles (Pt/C) using rotating disk electrode (RDE) measurements. The study corrects for noncompensated solution resistance and hydrogen mass transport to fit the kinetic current densities to the Butler–Volmer equation, allowing the determination of HOR/HER exchange current densities and mechanisms in alkaline solutions. In alkaline electrolytes, the HOR/HER rates on Pt are limited by hydrogen diffusion, making it difficult to quantify the kinetics using conventional RDE measurements. In contrast, in 0.1 M HClO₄, the HOR/HER rates are limited entirely by hydrogen diffusion, which prevents accurate quantification of the kinetics. The study shows that the HOR/HER kinetics on Pt in alkaline electrolytes are significantly slower than in acid, leading to higher anode potential losses in alkaline fuel cells for low platinum loadings. This is in contrast to proton exchange membrane fuel cells (PEMFCs), where the HOR kinetics are fast and do not significantly affect the cell voltage. The study also compares the ORR activity of Pt(pc) and Pt/C in both acid and alkaline electrolytes. The ORR activity of Pt/C is found to be lower than that of Pt(pc) in alkaline solutions, indicating a Pt particle-size effect on the ORR kinetics. The ORR activities in acid and alkaline solutions are found to be similar, suggesting that the ORR overpotential on a Pt cathode in alkaline fuel cells (AFCs) would be similar to that in PEMFCs under the same conditions. The study concludes that the HOR/HER kinetics on Pt in alkaline electrolytes are several orders of magnitude slower than in acid, which has significant implications for the performance of AFCs/AMFCs. The use of Pt/C anode catalysts in AFCs/AMFCs would require high loadings, making them a significant cost factor. However, the use of Pt/C cathode catalysts in AFCs/AMFCs would result in comparable ORR potential losses as in PEMFCs, but other non-noble metal-based cathode catalysts are available for AFCs/AMFCs with ORR activity comparable to Pt/C. Therefore, the development of highly efficient catalysts for HOR in alkaline electrolytes is a critical challenge to make AFCs/AMFCs more practical.This study investigates the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) kinetics on platinum (Pt) in both acid and alkaline electrolytes. The research focuses on polycrystalline platinum (Pt(pc)) and high surface area carbon-supported platinum nanoparticles (Pt/C) using rotating disk electrode (RDE) measurements. The study corrects for noncompensated solution resistance and hydrogen mass transport to fit the kinetic current densities to the Butler–Volmer equation, allowing the determination of HOR/HER exchange current densities and mechanisms in alkaline solutions. In alkaline electrolytes, the HOR/HER rates on Pt are limited by hydrogen diffusion, making it difficult to quantify the kinetics using conventional RDE measurements. In contrast, in 0.1 M HClO₄, the HOR/HER rates are limited entirely by hydrogen diffusion, which prevents accurate quantification of the kinetics. The study shows that the HOR/HER kinetics on Pt in alkaline electrolytes are significantly slower than in acid, leading to higher anode potential losses in alkaline fuel cells for low platinum loadings. This is in contrast to proton exchange membrane fuel cells (PEMFCs), where the HOR kinetics are fast and do not significantly affect the cell voltage. The study also compares the ORR activity of Pt(pc) and Pt/C in both acid and alkaline electrolytes. The ORR activity of Pt/C is found to be lower than that of Pt(pc) in alkaline solutions, indicating a Pt particle-size effect on the ORR kinetics. The ORR activities in acid and alkaline solutions are found to be similar, suggesting that the ORR overpotential on a Pt cathode in alkaline fuel cells (AFCs) would be similar to that in PEMFCs under the same conditions. The study concludes that the HOR/HER kinetics on Pt in alkaline electrolytes are several orders of magnitude slower than in acid, which has significant implications for the performance of AFCs/AMFCs. The use of Pt/C anode catalysts in AFCs/AMFCs would require high loadings, making them a significant cost factor. However, the use of Pt/C cathode catalysts in AFCs/AMFCs would result in comparable ORR potential losses as in PEMFCs, but other non-noble metal-based cathode catalysts are available for AFCs/AMFCs with ORR activity comparable to Pt/C. Therefore, the development of highly efficient catalysts for HOR in alkaline electrolytes is a critical challenge to make AFCs/AMFCs more practical.