Measurements of CH2O, HNO2, O3, NO2, and SO2 at four different sites in western Europe are reported. At the maritime sites (Loophead, Ireland, and Dageb¿ll, Germany) the mixing ratios of the pollution indicators SO2 and NO2 are <0.5--2 ppb, while concentrations of several tenths of a part per billion of CH2O are consistent with photochemical equilibrium. HNO2 was only found at continental sites (J¿lich and Deuselbach) with SO2 and NO2 mixing ratios up to 100 ppb. Surprisingly, NO3 was found to be at least 10 times lower than expected in most cases.