Several PEM fuel cell systems are currently being marketed or developed as natural gas fuelled stationary power generation or cogeneration systems. Whilst each of these may perform adequately when fuelled with methane, the composition of natural gas varies widely around the world, according to the source and even the season. This study investigated the variation in composition of fuel gas supplied in pipeline systems around Australia and how its chemical composition may affect the performance of PEM fuel cell systems than employ steam reforming, shift and selective preferential oxidation to convert the fuel gas to hydrogen. The study showed that the performance of the fuel cell system is affected by the chemical composition and that ammonia formed from gases with high nitrogen concentrations could limit the applicability of PEM fuel cells, and should be taken into account when manufacturers specify the allowable fuel gas compositions for their systems. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.