The use of microwave energy to facilitate sample decomposition prior to elemental analysis is now receiving considerable attention. Both wet and dry digestions are achievable. When microwave energy is used in combination with acid mixtures in closed vessels, the combined pressure and rapid heating can reduce digestion times to a few minutes from hours or days that may be required for open beaker digestions. This savings in time and labor is significant and has prompted the Office of Solid Waste to evaluate this technology as a preparation tool for solid wastes. Of particular interest are used oils and other fuels slated for incineration, incinerator ash, and particulates from these processes. This study reports on the evaluation of a commercially available microwave oven sample preparation system for this application. The effect of sample preparation conditions, including the acid matrix, heating time, and pressure, were evaluated for fifteen toxic or hazardous elements in particulates, ashes oils, and oil fuels. Analyses were carried out by inductively coupled plasma emission spectroscopy.