Two cruises were used to investigate the effect of the Orinoco River on the photochemistry of the eastern Caribbean in 1988. Hydrogen peroxide distribution was used as an indicator of the extent of modification and persistence of photochemical activity, with comparisons being made between high (fall) and low (spring) river input. During the spring cruise, H2O2 profiles were typical of oligotrophic waters, with accumulation rates and steady state concentrations similar to those reported for open ocean. Results from the fall cruise were characterized by increased surface concentrations, sharper gradients, and shallower penetration of the photochemically generated H2O2. The impact of the river water can be traced in the eastern Caribbean by increased dissolved organic matter fluorescence, lower salinity, and faster hydrogen peroxide accumulation rates in surface samples (up to 20 times greater for some stations). It is apparent that peroxide distribution throughout the eastern Caribbean was significantly affected by the Orinoco river input during the fall. ¿ American Geophysical Union 1993