The heterogeneous processes responsible for fog water chemical composition are examined in this paper. SO2/SO4= and NOx/HNO3/NO3- systems have been particularly investigated for their role in the production of acidity within the dispersed liquid phase. A program is presented, involving several European and American institutes, which has thus far provided the following main results: (1) nucleation scavenging represents a major source of sulphate in fog, but a potential also exists for SO2 conversion in the liquid phase; (2) excess nitrate which was found in fog, with respect to the prefog aerosol concentration, appears to derive from gas phase HNO3 incorporation into the droplets. Chemical processes seem to be active at the stage of fog formation, with microphysical condensation/evaporation/deposition/entrainment processes being primarily responsible for changes in the chemical composition of the system during fog evolution. Additional and improved data requirements indicated by our results are discussed. ¿ American Geophysical Union 1988