Hydrogen liquefaction systems have been the subject of intense investigations for many years. Some established gas liquefaction systems, such as the precooled Linde-Hampson systems, are not used for hydrogen liquefaction in part because of their relatively low efficiencies. Recently, more promising systems employing the modified Collins cycle have been introduced. This paper reports on second law analyses of a hydrogen liquefier operating on the modified Collins cycle. Two different modifications employing the cycle in question were attempted: (1) a helium-refrigerated hydrogen liquefaction system and (2) a hydrogen-refrigerated hydrogen liquefaction system. Analyses were carried out in order to identify potential areas of development and efficiency improvement. A computer code capable of computing system and component efficiencies; exergy losses; and optimum number and operating conditions of compressors, expanders, aftercoolers, intercoolers. and Joule-Thomson valves was developed. Evaluation of the thermodynamic and transport properties of hydrogen at different temperature levels was achieved by employing a hydrogen property code developed by researchers at the National Bureau of Standards (currently NIST). A parametric analysis was carried out and optimal decision rules pertaining to system component selection and design were reached. Economic analyses were also reported for both systems and indicated that the helium-refrigerated hydrogen liquefier is more economically feasible than the hydrogen-refrigerated hydrogen liquefier. Copyright (C) 2001 John Wiley & Sons, Ltd.