This study investigates the effect of nickel speciation and its equilibrium kinetics on the nickel toxicity to methylotrophic methanogenic activity. Toxicity tests were done with anaerobic granular sludge in three different media containing variable concentrations of complexing ligands. A correlation between nickel toxicity and the free nickel concentration failed, because not the equilibrium conditions, but the kinetics of the speciation processes taking place in the medium (precipitation, sorption, liquid speciation, etc.) determine nickel bio-uptake and its toxic effect. The latter was confirmed with an F-test (p-value always lower than 0.1). It was shown that the biological activity (methane production) took place within 3-20 days upon the start of methanogenic experiments, i.e. prior the chemical-physical equilibrium of nickel speciation was established in the methanogenic medium (10-20 days). The process of nickel sorption in the methanogenic granular sludge was limited by intra-particle diffusion and the experimental data fitted to the Weber-Morris sorption model. The other sorption kinetic models applied (pseudo-first order sorption kinetics, pseudo-second order sorption kinetics and first order reversible reaction kinetics) did not fit the experimental data satisfactorily. (C) 2010 Elsevier B.V. All rights reserved,