The speciation of Ni emissions from residual oil-fired utility boilers requires investigation because the possible presence of small respirable particles containing Ni3S2 is 8 health concern. An experimental approach was used to investigate the Ni speciation of residual oil combustion ash. Ash from a low- and high-S (0.33 and 1.80 wt %, respectively) residual oil was produced using a 42-MJ/h combustion system at excess O-2 concentrations of less than or equal to 1 and 2 or 3 mol %. Ni speciation analyses were performed using X-ray absorption fine-structure (XAFS) spectroscopy and sequential extraction-anodic stripping voltammetry (ASV). XAFS measurements indicate that >95% of the total Ni (3-9 wt %) present in the ashes occurs as Ni2+ coordinated to O2-. Both methods indicate that NiSO4 is the dominant form, although significant proportions of NiO (5-24%) were measured by sequential extraction-ASV. The sequential extraction-ASV method also indicated the presence of very small proportions, <2%, of Ni sulfide (NixSy). Fuel S content did not significantly affect Ni speciation; however, increasing excess O-2 concentrations promoted Ni sulfation. The predominance of NiSO4 and relatively low proportions of NixSy, in the experimentally produced ashes are inconsistent with sequential extraction-ASV determinations of Ni speciation in ashes collected from full-scale utility boilers. Differences in sampling procedures and combustion conditions are being investigated to explain this inconsistency.