The large-scale MARKAL Linear Program is applied (separately) to the energy systems of Québec and of Ontario, under several alternative constraints on emissions of CO₂, SO₂ and NO₂. A cost/emission, trade-off curve is derived for each province, and the main system impacts of the constraints are discussed and compared for the 2 provinces. Two main economic scenarios are investigated, with high and low economic growths, respectively. In addition, several sensitivity analysis scenarios are simulated. The results show, among other things, that the cost of CO₂ reduction is very sensitive to economic growth (as expected), and to such special constraints as a moratorium on nuclear (in Ontario) or on hydro (in Québec).