Ash from a low- and high-S (0.33 wt% and 1.80 wt% S, respectively) residual oil was produced using a laboratory-scale combustion system at excess O-2 concentrations of less than or equal to 1 and 2 or 3 mol%. High-S ashes are distinguished from low-S ashes by an abundance of (Na, K)(x) V-x(4+) V-6-x(5+) O-15 (0.90 > x > 0.54) and lack of (Na, K)(2)SO4 crystals. Discrete phases of Ni or Cr were not detected using SEM and XRD, even though these metals are relatively abundant-1.5 to 5.5 wt% and 0.08 to 0.1 wt%, respectively. Ni and Cr K-edge XAFS spectroscopy analyses indicate that NiSO4 and Cr-2(SO4)(3) and not the more toxic Ni3S2 and Cr6+ forms predominate in the ashes. Thermodynamic modeling results support the empirical results in that NiSO4. XH2O and Cr-2(SO4)(3) are predicted to be stable low-temperature species in both low- and high-S residual oil ashes produced at less than or equal to 3 mol% excess O-2.