Vertical profiles of H2O2 mixing ratios were obtained for each season from a site in central Arkansas during 1988. Aircraft-based measurements indicated that H2O2 mixing ratios followed an annual cycle, peaking during the summer at >6 parts per billion by volume (ppbv). The minimum occurred in winter when mixing ratios for H2O2 averaged about 0.2 ppbv. The H2O2 mixing ratio generally peaked at an altitude of about 800 mbar (2 km), although there may have been some seasonal dependence. The annual cycle followed variations in solar intensity, water mixing ratio, and temperature. Within a season, strong variations could be related to meteorological events. A daily cycle was inferred in which the H2O2 mixing ratio varied by a factor of 2 to 3; the peak observed values were at night. H2O2 mixing ratios at altitudes higher than 0.7 km were generally greater than local SO2 values above 0.7 km during all but the winter season. ¿ American Geophysical Union 1992