Combined external mass transfer and intrinsic biodegradation effects on the observed biodegradation rate were investigated for ferrous (II) cyanide complex (ferrocyanide) ions removal with calcium-alginate gel immobilized Pseudomonas fluorescens in a packed bed column reactor. Assuming first-order biodegradation kinetics, observed first-order biodegradation rate constants (k(p)) were calculated at different flow rates. To investigate the effect of external film diffusion on the observed biodegradation rate, several external mass transfer correlation models of the type J(D) = KNRe-(1-n) were tested and the mass transfer coefficients (k(l)) were calculated as a function of the mass flux (G) and the Reynolds number (N-Re) at different K and n values. The intrinsic first-order biodegradation rate constants (k) and the surface areas per unit weight of dried cells for mass transfer (a(m)) were determined from ilk, versus 1/G(n) plots at the same n values. Combining the k and k,a, values, the observed first-order biodegradation rate constants (k(p)) were calculated again at the same n values and compared with the k, values determined from experimental data. The mass transfer correlation J(D) Science Ltd. All rights reserved.