This study investigates the kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on polycrystalline platinum (Pt(pce)) and high surface area carbon-supported platinum nanoparticles (Pt/C) in 0.1 M KOH using rotating disk electrode (RDE) measurements. The noncompensated solution resistance and hydrogen mass transport were corrected from ac impedance spectroscopy, and the kinetic current densities were fitted to the Butler–Volmer equation with a transfer coefficient of α = 0.5. The results show that the HOR/HER rates on Pt/C electrodes in alkaline solution are much faster than those on Pt(pce) electrodes, making conventional RDE measurements insufficient for determining the kinetics. The specific exchange current densities of the HOR/HER on Pt(pce) and Pt/C were found to be comparable, with activation energies of approximately 29 kJ/mol. The HOR/HER kinetics in acid electrolytes are orders of magnitude faster and cannot be quantified by RDE measurements due to the Nernstian diffusion overpotential. The slow HOR kinetics in alkaline electrolytes are expected to cause significant anode potential losses in alkaline fuel cells, particularly for low platinum loadings. The ORR activity of Pt/C in alkaline solution is lower than that of Pt(pce), indicating a particle-size effect. The study concludes that the development of highly efficient catalysts for HOR in alkaline electrolytes is crucial for making AFCs/AMFCs more practical.This study investigates the kinetics of the hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER) on polycrystalline platinum (Pt(pce)) and high surface area carbon-supported platinum nanoparticles (Pt/C) in 0.1 M KOH using rotating disk electrode (RDE) measurements. The noncompensated solution resistance and hydrogen mass transport were corrected from ac impedance spectroscopy, and the kinetic current densities were fitted to the Butler–Volmer equation with a transfer coefficient of α = 0.5. The results show that the HOR/HER rates on Pt/C electrodes in alkaline solution are much faster than those on Pt(pce) electrodes, making conventional RDE measurements insufficient for determining the kinetics. The specific exchange current densities of the HOR/HER on Pt(pce) and Pt/C were found to be comparable, with activation energies of approximately 29 kJ/mol. The HOR/HER kinetics in acid electrolytes are orders of magnitude faster and cannot be quantified by RDE measurements due to the Nernstian diffusion overpotential. The slow HOR kinetics in alkaline electrolytes are expected to cause significant anode potential losses in alkaline fuel cells, particularly for low platinum loadings. The ORR activity of Pt/C in alkaline solution is lower than that of Pt(pce), indicating a particle-size effect. The study concludes that the development of highly efficient catalysts for HOR in alkaline electrolytes is crucial for making AFCs/AMFCs more practical.