Here we consider cost-effective solutions of emission control measures in Finland and the nearby areas of Estonia, St. Petersburg region, Karelia and Kola, in order to limit the acidifying deposition in Finland. In the study, the emission control costs of SO2, NOx and NH3 are assessed for the areas studied and an optimisation model developed for calculation of cost-optimal deposition control policies. The input data of the model consist of the cost functions describing the emission control costs to achieve lower emission levels for the gases and areas considered and of dispersion coefficients which describe the deposition due to an emission source in the deposition receptor grid squares. In addition, the model includes a description to calculate the acidifying load. The optimisation is based on linear programming. When the acidifying load of Southern Finland is reduced by minimising the total control costs, approx. three quarters of the total control costs are due to measures in the nearby areas, Estonia, St. Petersburg region, Karelia and Kola, and approx. one quarter due to measures in Finland. The distribution of costs in the cost-optimised cases depends relatively little on the level to which the acidifying load due the source areas considered are required to be reduced. If the load reduction target is moderate, the emission control measures should mainly be allocated to sulphur emissions and to some extent to ammonia emissions and, if the load reduction target is stricter, also to the emissions of nitrogen oxides.