Systematic trends observed in the polymeric hydrolysis constants of U(VI), Np(VI) and Pu(Vl) were analyzed by a simple hard sphere model, in which not the formal but the effective charges of actinide ions were considered. The parameter values such as the effective charge of AnO(2)(2+) were determined by fitting the reported hydrolysis constant data to the model. Using obtained parameter values, some predictions were made for unknown values of hydrolysis constants. (c) 2005 Elsevier B.V. All rights reserved.