Gaseous and particulate emissions for a heavy-oil-fueled power plant in Hungary were studied. Since fly ash is a heterogeneous mixture of different particle types, average compositions obtained by bulk analytical methods are not characteristic for the deposition and toxicological studies of trace elements. Therefore single particles obtained from both the flue gas and the electrostatic precipitator were analyzed by a variety of microanalytical methods. The usefulness of the single-particle electron probe microanalysis (EPMA) is shown for characterizing the different particle types of oil fly ash. The application of the morphological and chemical parameters obtained by this method in the modeling of the vanadium deposition due to oil fly ash emission is demonstrated. The in-particle inhomogeneities were studied by simultaneous microscopic X-ray fluorescence and X-ray diffraction (mu-XRF/mu-XRD), using synchrotron radiation. The results show that the oil fly ash particles are extremely heterogeneous, and a few "hot spots" having orders of magnitude higher Fe or elevated K and As content than the rest of the particles, The diffraction patterns of these sites indicate polycrystalline structure.