Spring/summer surface ozone concentrations, [O-3], in coastal environments were investigated: (1) by comparison of coastal and inland monitoring stations with data from a small island >5 km off the coast of southwest Sweden, (2) as a gradient from the coast towards inland in southernmost Sweden. Further, results from the chemical transport model MATCH were used to assess the marine influence on [O-3]. It was hypothesised that [O-3] is higher on the small island compared to the coast, especially during night and in offshore wind. Another hypothesis was that [O-3] declines from the coast towards inland. Our hypotheses were based on observations that the deposition velocity of O-3 to sea surfaces is lower than to terrestrial surfaces, and that vertical air mixing is stronger in the marine environment, especially during night. The island experienced 10 % higher [O-3] compared to the coast. This difference was larger with offshore (15 %) than onshore wind (9 %). The concentration difference between island and coast was larger during night, but prevailed during day and could not be explained by differences in [NO2] between the sites. The difference in [O-3] between the island and the inland site was 20 %. Higher [O-3] over the sea, especially during night, was reproduced by MATCH. In the gradient study, [O-3] declined from the coast towards inland. Both [O-3] and [NO2] were elevated at the coast, indicating that the gradient in [O-3] from the coast was not caused by NO titration. The conclusions were that surface [O-3] in marine environments is higher than in coastal, and higher in coastal than inland areas, especially during night.