A newly synthesized water-soluble phenylenediamine derivative, N-methyl-N-phenyl-3-sulfophenylenediamine sodium salt, proved to be of value as a reagent for the colorimetric determination of hydrogen peroxide in clinical analyses. The compound reacted with Trinder’s reagents to give chromogens with a higher sensitivity, a longer wavelength (>750 nm) as well as a higher water solubility compared with a commonly used reagent.