This paper reports the use of an advanced Multi-region Bottom-up model, the Extended MARKAL, for an in-depth investigation of the responses by the Québec-Ontario energy system to a series of increasingly severe Greenhouse gas emission reduction targets. For each target, the responses are analyzed under four policies resulting from the adoption or not of a joint emission target and of electricity exchanges. Results indicate significant cost savings and a reduction in the need for nuclear energy in Ontario, which suggests that cooperative responses to GHG emissions caps should be seriously considered by the two provinces. The explanation of complex systemic responses underscores the advantages of using detailed bottom-up models. Results are highly credible and of immediate relevance to the policy makers. The present analysis can be readily extended to at least four Canadian provinces for which separate MARKAL models already exist (i.e. Alberta and Saskatchewan, in addition to Québec and Ontario).