In our previous paper, we reported a hydrogen peroxide sensor that used crude particles of MnO2 immobilized in a polyvinyl chloride membrane attached on top of a dissolved oxygen sensor (Nagata et al., 2000). In this study we used MnO2 particles of under 400 mesh for the sensor preparation. With this sensor, the effects of the coexistence of oxidizing substances and of various water preparations on the steady-state sensor response, R, were investigated. The results were as follows. 1) The use of fine MnO2 particles of under 400 mesh improved the sensor sensitivity for H2O2 in distilled water to more than twice that of the sensor reported previously. The temperature dependency increased about 10% per 10 K as the temperature increased from 288 to 308 K, with the sensitivity at 298 K as 100%. 2) R-s values for oxidizing substances dissolved in distilled water were negligibly small. The coexistence of oxidizing substances with H2O2 in distilled water affected R-s values by less than 4%. For H2O2 dissolved in tap water, NaClO aqueous solution, or chlorine water, the R-s values were 4 to 11% lower than that for the distilled water solution of H2O2. For seawater solution of H2O2, the R-s values were almost equal to that of the distilled water solution of H2O2.