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Surface ozone data from 25 European low-altitude sites and mountain sites located between 79 deg N and 28 deg N were studied. The analysis covered the time period March 1989-February 1993. Average summer and winter O3 concentrations in the boundary layer over the continent gave rise to gradients that were strongest in the north-west to south-east direction and west-east direction, respectively. Wintertime O3 ranged from 19 to 27 ppb over the continent, compared to about 32 ppb at the western border, while for summer the continental O3 values ranged between 39 and 56 ppb and the oceanic mixing ratios were around 37 ppb. In the lower free troposphere average wintertime O3 mixing ratios were around 38 ppb, with only an 8 ppb difference between 28 deg N and 79 deg N. For summer the average O3 levels decreased from about 55 ppb over Central Europe to 32 ppb at 79 deg N. In addition, O3 and Ox (= O3 + NO2) in polluted and clean air were compared. The amplitudes of the seasonal ozone variatio ns increased in the north-west to south-east direction, while the time of the annual maximum was shifted from spring (at the northerly sites) to late summer (at sites in Austria and Hungary), which reflected the contribution of photochemical ozone production in the lower parts of the troposphere.