Combined biodegradation and internal diffusion effects on the biodegradation rate of ferrous(II) cyanide complex (ferrocyanide) ions by Ca-alginate gel immobilized Pseudomonas fluorescens beads were investigated as a function of initial ferrocyanide concentration and particle size in a batch system. Assuming first-order biodegradation kinetics (v = kC), first-order biodegradation rate constants for free and different sized immobilized particles were predicted and at 100 mg l (1) bulk ferrocyanide ion concentration experimental effectiveness factors (eta) were determined for each particle size. Then using these data, the Thiele modulus (phi) was evaluated for each particle size. Finally effective diffusion coefficient (D-e) was calculated from the Thiele modulus equation, which is a function of particle size. effective diffusion coefficient and first-order biodegradation rate constant. The results showed that the intraparticle diffusion resistance has a significant effect on the observed biodegradation rate. (C) 2002 Elsevier Science Ltd. All rights reserved.