Measurements were carried out to determine the seasonal and diurnal variations of the size distribution of sulfate particles. The results of these measurements show that there is an unambiguous seasonal variation which is caused by the changes of the relative humidity of the air. On the other hand no diurnal variation was found because during the sampling time the relative humidity was smaller than the value necessary for the phase transition of the particles. The measured growth with increasing relative humidity of the mass median radius of sulfate particles is practically comparable with the theoretical growth of a pure ammonium sulfate particle having a 0.14 μ dry radius. On the basis of these findings the mechanism of the sulfate particle formation is briefly discussed.