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= KN Re −(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 1/ k p versus 1/ G n plots at the same n values. Combining the k and k l a m values, the observed first-order biodegradation rate constants ( k p) were calculated again at the same n values and compared with the k p values determined from experimental data. The mass transfer correlation J D=1.625 N Re −0.507 predicted accurately the experimental data.