This work compares two classes of exergy efficiency definitions and applies them on a simple process (i.e. PRICO) for the liquefaction of natural gas. This comparison demonstrates the possible inconsistency and uncertainty of several exergy efficiency definitions already proposed in the literature. In addition, the paper highlights the importance of decomposing the total exergy into its relevant components, and to analyze the behavior of these exergy components individually. The exergy of heat as well as the exergy of a material stream exhibit a special and different behavior below ambient temperature compared to above ambient conditions. This is illustrated by calculating exergy efficiencies for the PRICO process for liquefaction of natural gas, and the results also show different figures for the various exergy efficiencies proposed in the literature. (c) 2012 Elsevier Ltd. All rights reserved.