The dissolution of nickel oxide (bunsenite) in acid solutions containing oxalic acid has been studied at 70.0 degreesC. The dependencies of the rate of dissolution on total oxalate concentration and on pH have been explained by assuming a mechanism involving the transfer of two different surface complexes, I and II, that predominate in different pH ranges. The rate law is R = k(1){I}+k(2){II},where {} denotes surface concentration. The values k(1)N(s) = 3.04 x 10(-3) mol Ni m(-2) s(-1), k(2)N(s) = 1.84 x 10(-3) mol Ni m(-2) s(-1), together with the stability constants K-1 = 675 mol(-1) dm(3) and K-2 = 60 mol(-1) dm(3) fit all the results very well. The species formed in more acidic media is both more stable and more reactive. The possible structures of I and II are discussed on the basis of Fourier transform IR spectroscopy evidence in the literature.