Polymer electrolyte membrane (PEM) fuel cell stacks to 1 kW(e) capacity, with an active area of 225 cm(2) per cell, have been constructed and operated to investigate the fuel quality issues (one of the major barriers for commercialization of this technology), and start/stop, thermal cycling and load following capabilities. The stacks were assembled and tested in stages of 2-, 4-, 8- and 15-cell configurations. This paper describes the design and assembly of the stacks tested, analysis of the results and problems encountered during operation. Though the 1 kW(e) stack showed a large variation in the temperature of the interconnect plates due to uneven cooling, the individual cell voltages were found to be within 86 mV (under full load). The average power produced by each cell for the 1 kWe stack operating on air/H-2 was 67.5 W (300 mW cm(-2)). The stack has undergone more than 40 cold start/shut down thermal cycles in the power output range of 0.6-1 kW, over an accumulated operation of similar to300 h with a small degradation in its performance. The electrical efficiency of the stack varied from 39 to 41%. The recoverable combined heat and power (CHP) efficiency of the stack was 65% without external thermal insulation and 80% with external thermal insulation. (C) 2003 Published by Elsevier B.V.