Measurements of H2O2, both in the gas phase and in cloudwater samples, were made over central Ontario during the summer 1988 and spring 1990 intensive measuring periods of the Eulerian Model Evaluation Field Study from a Twin Otter aircraft. Cloudwater concentrations range from less than 1.5 μM to greater than 100 μM; corresponding gaseous concentrations range from less than 0.1 parts per billion by volume (ppbv) to approximately 2.5 ppbv. Gas phase H2O2 data are corrected for instrument response and the interphase partitioning of H2O2 is examined. Ratios of the measured to predicted partitioning based on Henry's law equilibrium range from approximately 0.2 to 1. When the data correction is applied, most samples do not differ from equilibrium by more than 20%. Lower ratios are associated with samples from supercooled clouds or samples taken when interstitial SO2 is high. The measured aqueous fraction for the supercooled samples is approximately 20% of that expected by Henry's law equilibrium. The ratios are independent of liquid water content but show some dependence on pH. ¿ American Geophysical Union 1995