The second-law efficiency is proposed for measuring and comparing the performance of electrochemical processes. It measures how close the process approaches a reversible process. Other definitions of efficiency found in the literature based on the first law of thermodynamics can generate efficiency values that are >100% for certain systems depending on whether the change in entropy for the overall chemical reaction involved in the process is positive or negative. Such efficiency values are misleading while making it difficult to compare processes that absorb or release significant quantities of thermal energy. Use of the proposed efficiency which is within the frame work of the second law of thermodynamics, can reach 100% only in the limit of a reversible process and will always stay <100% whether the entropy change is positive or negative. Furthermore, it accounts for the quality of thermal energy added to or removed from the system and thus provides a more consistent way of comparing different types of electrochemical devices. This consistency will allow better comparison of performance on a widespread basis, i.e., between groups working on distinct electrochemical systems. (C) 2004 Elsevier B.V. All rights reserved.