Gaseous hydrogen peroxide (H₂O₂) concentrations were measured in the city of Nagoya during the period from January 1993 to August 1994. The results show both diurnal and seasonal variations together with a special event. They reached the maximum concentrations in the afternoon mainly due to photochemical reactions in the polluted atmosphere, and the minimum concentrations were observed at mid-night. Daytime H₂O₂ concentrations were principally correlated with the solar radiation, whereas night-time H₂O₂ concentrations were well correlated with wind speed as a, n indication of boundary layer instability. Seasonally, the maximum concentrations appeared in summer and the concentrations were very low in winter. The monthly mean value reached the maximum in May in 1993, but in August in 1994. This shows that there was a depletion of solar radiation during July and August in 1993. Judging from SO₂ concentrations measured in the city of Nagoya, there is a severe shortage of the reactive oxidant to SO₂ in winter. A dramatic phenomenon that the ground level H2O2 concentrations in night-time abruptly increased in association with the passage of a cyclone was observed. This is likely due to direct downward transport of H₂O₂ from the free atmosphere where H₂O₂ concentration is usually higher.