The distribution, production, and decay of hydrogen peroxide (H2O2) were studied in 10 boreal lakes of differing physical-chemical characteristics. Diurnal and vertical fluctuations in H2O2 concentration were followed in the lakes by sampling at six depths three times per day. In addition, incubations of water filtered through 0.2-mu mesh were made under artificial irradiation to study the abiotic production and decay of H2O2. H2O2 concentrations after 8 h of artificial irradiation were significantly correlated with neither absorption coefficients at 320 nm nor with dissolved organic carbon (DOC) concentrations. However, H2O2 concentration increased rapidly with DOC concentration among lakes with DOC concentrations below 10 mg.L-1. The H2O2 concentration after exposure to ultraviolet radiation was positively related to the half-life of H2O2, which in turn was negatively correlated with iron and manganese concentrations (r(2) = 0.68 and 0.70, respectively). In situ H2O2 concentrations at the surface of the water column ranged between 30 and 1041 nmol.L-1 and were largely determined by the decay rates of H2O2 and the mixing depth of the water column.