We have developed a vertical-flux model of undersea irradiance (300--500 nm) using measured absorption, scattering, midday spectral irradiance, and midday vertical attenuation (Kd) for stations in the eastern Caribbean in 1988. The model varies the pathlength and geometry of undersea light with solar zenith angle according to scattering in the water column, and according to the ratio of direct sunlight to diffuse skylight. Modeled UV and visible Kd have diel variations up to 67%, which are greatest above 400 nm. Using an experimentally determined photoproduction quantum yield (normalized to total dissolved organic matter absorption) which decreases exponentially as wavelength increases, modeled H2O2 photoproduction is primarily due to 320--360 nm irradiance. A significant fraction of total production occurs above 400 nm, especially at depth and at smaller zenith angles. The diel distribution of H2O2 photoproduction is notably deeper at midday than would be predicted by UV excitation in a simple nonscattering model. ¿ American Geophysical Union 1993