Chemical speciation of nickel is necessary for risk assessment of foss
il fuel fly ash. We have used a standard coal fly ash in sequential ex
traction to show reproducibility and mass balance of nickel phase frac
tionation, and applied it to two field samples of oil fly ash. Nine di
stinct nickel phases were quantitated by voltammetry and characterized
by cumulative extraction plots and dissolution kinetics. They are fur
ther correlated with SEM/EDX analysis of the fly ash. This nickel spec
iation method reflects the difference in the combustion process in gen
erating the two oil fly ash and allows comparison of their nickel dist
ribution and bioavailablity.