Hydrogen peroxide (H(2)O(2)) is the most stable of the reactive oxygen species produced by photochemical reactions in natural waters. To relate H(2)O(2) concentrations to solar irradiance, we made daily measurements of H(2)O(2) in the Water of Leith, a freshwater stream in Dunedin, New Zealand, and co-located continuous measurements of the intensity of solar radiation, from September 2003 to March 2006. A simple model in the form of a first-order differential equation was fitted to the measurements. The model describes the H(2)O(2) concentrations over time by using photochemical production rates from ultraviolet-B (UVB), UVA and photosynthetically active radiation (PAR), and loss rates from temperature-dependent and temperature-independent processes. The retrieved model terms confirmed that H(2)O(2) is produced by both UVB and UVA radiation. These results demonstrated that changes in solar radiation reaching the study site were closely correlated with the observed seasonal pattern in H(2)O(2) concentrations in the water.