A concentration/deposition model has been developed for application to the Long-Range Transport of Air Pollutants (LRTAP) program within the Canadian Department of the Environment. The Lagrangian model computes concentration and deposition fields of sulphur compounds as well as precipitation pH over North America. Four-day, backward trajectories are computed from three-dimensional objectively analyzed windfields, which are available 4 times a day. The pollutants are assumed well mixed throughout the mixing layer. The mixing height varies diurnally and seasonally according to mixing height climatologies. Precipitation data are obtained from 24 hour objective analyses and SO2 source emissions from a North American emission's inventory. Wet and dry deposition and chemical transformation processes are parametrized. Model predicted air and precipitation sulphate concentrations have been compared with those measurements by different monitoring networks in eastern North America. The agreement between measured and computed quantities is generally good. The occurrences of air pollution episodes are duplicated in space and time. Average computed and measured concentration/deposition fields are generally within 60%. The paper gives a description of the model, shows its sensitivity to changes in values of key parameters, and gives results from a comparison study in 1977. Further, some problems encountered when using actual measured data for model comparison are discussed.