The biodegradation kinetics of phenol including biodegradation and diffusion together was investigated in Ca-alginate-immobilized Pseudomonas putida beads as a function of particle size in a batch system. The effective diffusion coefficient of phenol in the bead was predicted assuming first-order biodegradation kinetics. For this purpose, the effect of phenol concentration (C-s) on the biodegradation rate (v) was investigated and the first-order biodegradation rate constants for free and different sized immobilized particles were determined from C-s vs. v plots. Using biodegradation rates, experimental effectiveness factors (eta) were determined at 100 mg/l bulk phenol concentration. Thiele modulus, which is a function of particle size, effective diffusion coefficient, and first-order biodegradation rate constant was evaluated from the experimental effectiveness factor and effective diffusion coefficient was calculated from the Thiele modulus formulae for each particle size. The results showed that intraparticle diffusion resistance has a significant effect on the phenol biodegradation rate.