The implementation of hybrid electric vehicles powered with alternative fuels is critical in reducing national dependence on imported crude oil, addressing the detrimental environmental impact of increasing petroleum usage worldwide, and sustaining the national economy. The question is not whether changes should be made, but instead centers on identifying pathways that will lead to the greatest environmental and economic benefits. To avoid misuse of limited infrastructure investment, the objective of this research is to consider a broad range of relevant factors to determine desirable power plant-fuel combinations for hybrid electric vehicles. In the long term, fuel cells may dominate this application, but at least in the short term, proton exchange membrane fuel cells (PEMFCs) will not likely offer immediate substantial benefit over internal combustion (IC) engines. Environmentally friendly operation of the PEMFC results partly due to low-temperature operation but primarily due to the requirement of a clean fuel, hydrogen. In addition, the differential benefits from power plant choice can be overshadowed by the advantages obtained from hybrid electric vehicle technology and alternative fuels. Consequently, the fuel flexibility of IC engines provides an advantage over the relatively fuel inflexible PEMFC. The methane/hythane IC engine hybrid option, as developed and presented here, is a promising pathway that avoids the barriers encountered with conventional non-hybrid natural gas vehicles, namely range, power and fueling infrastructure difficulties. Dynamometer testing of the natural gas hybrid prototype on the certification FTP-72 duty cycle revealed very low emissions and mileage greater than 33 miles per gallon gasoline equivalent. This hybrid option utilizes a domestic, cost-effective fuel with renewable sources. With multi-fuel capability (methane, hythane and gasoline) it is also designed for use within the emerging hydrogen market. This hybrid option offers reliability and cost-effective technology with immediate wide spread market availability. Copyright (C) 2007 John Wiley & Sons, Ltd.