The effect of substituents on the kinetics of flotation of a copper-nickel ore from Canada by N-arylhydroxamic acids was studied. Seven N-arylhydroxamic acids were synthesized and tested to float a nickel ore containing pentlandite (4-5%) chalcopyrite (4-5%) and pyrrhotite (30-35%). The time-recovery plots were fitted using the modified first order rate equation for flotation kinetics, namely, R(t) = R(infinity) e(-k(t + phi)). N-phenylacetyl-N-(2,6-dimethylphenyl)hydroxylamine (PANXHA) was found to have the highest first order rate constant for the kinetics of flotation of pentlandite and this was higher than that for potassium amyl xanthate (PAX). Amongst the seven N-arylhydroxamic acids, PANXHA was also found to have the highest selectivity index for pentlandite. Modified rate constants were linearly related to calculated dissociation constants (pK(a)) of the N-arylhydroxamic acids used in the study. (C) 2009 Elsevier B.V. All rights reserved.