During the Austral winter of 1995, a series of measurements were made as part of the Southern Ocean Atmospheric Photochemistry Experiment-1 (SOAPEX-1) campaign at the Cape Grim Baseline Air Pollution station (41°S) on the northwestern tip of Tasmania, Australia. The measurements were complimentary to similar ones made in the Austral summer during January–February 1995 and both sets of data are used to make a comparison of photochemistry in the remote marine boundary layer in summer and winter. Seasonal measurements of peroxy radicals are compared to both a steady-state analysis and a full model in order to elucidate their relationships to measured O3, Full-size image (<1 K) and H2O. Calculations of the seasonal concentrations and relationships of OH are consistent with both model and measurements. From a detailed analysis of the ozone budgets accounting for the contributions of photochemistry, deposition and entrainment processes, it is clear that on a seasonal basis there are two major controlling factors working in opposition. The net effect of photochemistry in the remote marine boundary layer is to destroy ozone; this is balanced by entrainment of ozone into the marine boundary layer from the lower free troposphere. Using the measured data, photochemistry represents the dominant loss process for ozone on a seasonal basis varying from 1.2 ppbv d−1 in summer (87% of the ozone loss) to 0.6 ppbv d−1 in the winter (64% of the ozone loss). It is clear that the level of photochemistry taking place in the marine boundary layer determines the lower bound for ozone levels.