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Sulfate aerosols are harmful as respirable particles. They also play a role as cloud condensation nuclei and have radiative effects on global climate. A combination of delta18O-SO4 data with catchment sulfur mass balances was used to constrain processes affecting S cycling in the atmosphere and spruce forests of the Czech Republic. Extremely high S fluxes via spruce throughfall and runoff were measured at Jezeri (49 and 80 kg S ha(-1) yr(-1), respectively). The second catchment, Na Lizu, was 10 times less polluted. In both catchments, delta18O-SO4 decreased in the following order: open-area precipitation > throughfall > runoff. The delta18O-SO4 values of throughfall exhibited a seasonal pattern at both sites, with maxima in summer and minima in winter. This seasonal pattern paralleled delta18O-H2O values, which were offset by -18 per thousand. Sulfate in throughfall was predominantly formed by heterogeneous (aqueous) oxidation of SO2. Wet-deposited sulfate in an open area did not show systematic delta18O-SO4 trends, suggesting formation by homogeneous (gaseous) oxidation and/or transport from large distances. The percentage of incoming S that is organically cycled in soil was similar under the high and the low pollution. High-temperature 18O-rich sulfate was not detected, which contrasts with North American industrial sites.