Two laboratory experiments on heterophase transformations leading to cloudwater acidification are described. The first concerns the SO 2 absorption from monodisperse water droplets without catalyzers. With gas-phase concentration [SO 2] g = (100± 4) ppmv at t = 25°C, the mean sulfur concentration in the droplets after contact time t =1×10 −2s is [ S(IV)] = (1.4±0.2)× 10 −3 M , i.e. [ S(IV)] = 0.95 [ S(IV)] e . These experimental results are compared with values calculated from theoretical models. In the second experiment the catalytic activity of carbon particles is examined for SO 2 oxidation, again on monodisperse water droplets. With [SO 2] g = (25±2) ppmv at t=25°C, the mean value for sulfate produced in solution catalyzed by C is: [SO 4 2−]=(0.56±0.1)× 10 −3 M after a contact time of 54 s. If we express the oxidation reaction as: d[SO 4 2−]/dt = k[C c][S(IV)] the reaction rate constant is k = (0.14±0.03) M −1 s −1 .