The obligations resulting from the Kyoto Protocol (KP) are implemented by many participants, e.g. Greece, by a combination of two methods: (a) application of regional measures which restrict CO 2 emissions and (b) procurement of green certificates e.g. in a Climate Exchange Market (CEX). Therefore, the cost for compliance with the KP depends on the extent each method is used and also on the traded values in the CEX. The energy policy and planning to be considered are long-term items and now extension of the KP to a post-KP is discussed which reaches year 2020. In Greece, the electricity sector is with a weighted CO 2 contribution of 73%, the dominantly emission sector. The paper analyzes the cost and other merits of different scenarios for the expansion of electrical power system in Greece. For different scenarios, the total cost of the electrical power system expansion is calculated as a function of the price of emission certificates. It has been shown that there is a price of 27.5h/tCO 2 , above which specific mitigation measures may lead to cost optimum solutions. A sensitivity analysis is also presented concerning the variation of key parameters like the participation in the system expansion of new supercritical coal units, the price evolution of natural gas, the RES usage rate and the discount rates of the expansion investments. Results may be of interest for a decision on the cost optimum electrical power system expansion.