Recent observations at shoreline sites downwind and over water from urban areas have shown enhanced ozone concentrations when the predominant wind direction is from the urban area to the site. Kinetic photochemical computations simulating this situation have been performed and are compared with similar calculations in which overwater transport is replaced by overland transport. The 'overwater effect' is reproduced by the calculations, giving an increase in the ozone maximum of ~7% for the conditions chosen. The effect is shown to result principally from an enhancement of the NO2/NO ratio during overwater transport, with a concomitant increase in ozone concentrations as a result. Another consequence of the overwater transport is increased conversion ratios in the chemical production chains of peroxyacetyl nitrate, nitric acid, and gaseous sulfate.