Fuel cells are being considered as notable new energy sources due to, not only their potential for obtaining high energy conversion efficiencies, but also their environmental sensitive features. These are especially important now that the problems relating to global environmental pollution are regarded as serious social issues. Polymer electrolyte fuel cells (PEFCs) in particular are being pursued due in part to the prospect of realizing timely enhancements to several key characteristics, including size and power. Encouraged by these anticipated improvements, PEFCs are being investigated as promising power generator candidates for hybrid electric vehicles. The substantial future potential of PEFCs has been noted by TOYOTA, and has motivated extensive R&D activities toward the practical application of PEFCs to hybrid electric vehicles. These R&D efforts include, not only activities on such key areas as performance enhancement, but also extensive attention to a broad range of related concerns, such as cost reduction, reformer development, system integration, durability, reliability and so on. From these diverse tasks, this paper focuses on the issues related to ensuring adequate PEFC performance when reformed fuel is utilized. Recent outcomes of R&D conducted at TOYOTA on this topic will be described.