Nanometallic iron and aluminium, along with hydrogen and electricity, are among the proposed alternatives to the petroleum-based fuels for future transportation. The advantages of the metallic fuels appear to be high volumetric energy densities and zero greenhouse gas emissions during the operation of the vehicle. However, nanometallic fuels do not exist in nature, and a well-to-wheel analysis of the fuel manufacture-utilization system is required to quantify the energy consumption and assess the true environmental impact of the proposed alternative. The three-component nanometallic fuel system consisting of a metal production process, a nanoparticle formulation process and the metal combustion process is analysed in this paper. The energy balance and the environmental impact are estimated for nanometallic iron and aluminium based systems. The sustainability of once-through systems that do not involve recycle of combustion products is questionable because of resource limitations. A viable system for satisfying the transportation fuel demands will involve the reduction and recycle of the combustion products. A comparison of these nanometallic fuels with gasoline and hydrogen indicates that nanometallic fuels are the least efficient, with primary energy consumption greater than 11 MJ km(-1) compared to 0.625 MJ km(-1) for gasoline and 8.6 MJ km(-1) for hydrogen. The nanometallic fuels will also have the most severe impact of the three, with CO2-equivalent emissions of 13.44 billion tons year(-1) for iron and 21.1 billion tons year(-1) for aluminium as compared to approximately 0.8 billion tons year(-1) for gasoline. These emissions from nanometallic fuels are at least an order-of-magnitude higher than those for gasoline and hydrogen. The results of the analysis emphasize the need for well-to-wheel assessment for determining the true impact of technologies proposed as replacements for the current technologies. Copyright (c) 2006 John Wiley & Sons, Ltd.