A reliable algorithm based on differential evolution (DE) for solving short-term combined economic emission scheduling of hydrothermal systems with several equality and non-equality constraints is presented in this paper. The water transport delay between connected reservoirs is also considered. The problem is formulated considering both cost and emission as competing objectives. Combined economic emission scheduling (CEES) is a bi-objective problem. A price penalty factor approach is utilized here to convert this bi-objective CEES problem into a single objective one. The effect of valve point loading is also taken into account in the present problem formulation. The effectiveness of the proposed method is tested on a sample test system consisting of four cascaded hydro units and three thermal units. The results of the proposed technique based on DE are compared with other population based method. It is found that the results obtained by the proposed technique are superior in terms of fuel cost, emission output. It is also observed that the computation time is considerably reduced by the application of the proposed technique based on differential evolution. (C) 2008 Elsevier Ltd. All rights reserved.