This paper describes the execution and analysis of one of a series of field experiments performed at Great Dun Fell in the northern Pennines of England designed to study the oxidation of SO2 in cloud. The experiments are designed to isolate the effect on cloud chemistry by artificially relasing SO2 gas into the entering cloud base. Observations made in cloud on April 30, 1986, clearly and unequivocally demostrated the production of sulphate ions in cloudwater. A simultaneous decrease in H2O2 concentration demostrated that about 40% of this arises from rapid reaction between H2O2 and SO2. The results of a comprehensive model of the cloud physical and chemical development at the site show good agreement with the observations, and indicate that the remaining fraction of sulphate produced resulted from the reaction of O3 with SO2 in cloudwater, promoted by the relatively high pH found in the cloud. Extrapolation of the results to other input gaseous concentrations by means of the model demostrates the importance of gaseous ammonia in controlling cloudwater pH and hence the oxidation of SO2 by O3, in that it is found that over a wide range of SO2 concentrations, the total sulphate produced by the time air reaches a given point in the cloud is virtually independent of the initial SO2 concentration, and roughly proportional to the ambient ammonia concentration. ¿American Geophysical Union 1990