Some recent laboratory results relating t o the conversion of gaseous sulphur dioxide and nitrogen dioxide into particulate material and the implication of these results on the atmospheric chemistry of these gases is discussed. Homogeneous oxidation of SO, by photochemically initiated reactions may occur at a rate ranging from 0.1 % hr-' in clean air t o a 0-10 % hr-1 in urban air. The oxidation rate of NO, is more raped (up t o 25% hr-l) but may be effectively less due t o the reversible nature of the reactions. Theoretical and experimental results suggest that the rate of formation of H,SO, in urban air (from SO,) is sufficient for heteromolecular nucleation t o form aqueous sulphuric acid aerosols but in background areas condensation on the existing aerosol is the main fate of the SO, oxidation products. Sufficient gaseous HNO, for nucleation is unlikely to be produced from NO, in the atmosphere and condensation on aerosol particles, particularly hygroscopic particles, is an important removal mechanism for HNO,. LL .arge effect of relative humidity on the rate of the nucleation and condensation processes is demonstrated experimentally.