X-ray absorption fine structure (XAFS) spectroscopy is a powerful non-destructive, direct technique for determining the speciation of environmentally important elements in products derived from combustion of fossil fuels. Such information is potentially important (i) for assessing the threat to human health posed by specific forms and oxidation states of such elements in combustion products (ash) or in combustion-derived airborne particulate matter (PM), and (ii) for possible source identification and apportionment in PM investigations. The specific examples discussed include the speciation of various elements classified as hazardous air pollutants (HAPs) in ash products from combustion of coal (As and Cr), residual oil (Ni and Cr), and biomass (Cd and Zn) and in airborne PM collected on a PM10 filter (S, Cl, Cr and As). Chromium and arsenic, which could exist in these materials in different oxidation states, were typically found predominantly in less toxic oxidation states, Cr(III) and As(V). All metal species (Cr, Ni, Cd, Zn, As) were shown to be present in the combustion ashes in predominantly oxidic environments (i.e., oxides, sulfates, arsenates, etc.). Most of the sulfur in the PM10 filter sample was present as sulfate, but minor organosulfur forms (thiophene) were also identified. For comparison with the data obtained for elements on the PM10 filter, XAFS data are also presented for the corresponding elements in two National Institute of Standards and Technology (NIST) particulate matter Standard Reference Materials (SRMs): Urban PM (SRM 1648), and Diesel PM (SRM 1650). (C) 2000 Elsevier Science B.V. All rights reserved.