Binary-type energy conversion systems are typically used to exploit low-temperature geothermal resources. There are many different technical variations of binary plants, including those known as Organic Rankine Cycles (ORC) and proprietary systems known as Kalina. cycles. Recent articles in the technical literature claim dramatic efficiency advantages for Kalina cycles over ORCs, thereby providing the motivation for this study. Claims of remarkable superiority for one type of technology relative to another ought to be verifiable by recourse to actual performance data. Since there is now an actual Kalina cycle in operation, it ought to be possible to make a comparison between it and ORC plants that have been in operation for some time. Comparisons between power plants must use an appropriate and consistent thermodynamic basis. It has been shown that the Second Law of thermodynamics is the best basis for such comparisons. This principle, employing the concept of exergy, is used here to shed light on these claims. Furthermore, we introduce a methodology to render the comparison of plant efficiencies on common input and environmental conditions, even though the plants being compared operate with somewhat different fluid inlet and ambient temperatures. (C) 2004 CNR. Published by Elsevier Ltd. All rights reserved.