An oxygen transfer model was established for Pichia pastoris growing on glycerol and methanol in a stirred tank bioreactor and expressing a recombinant human serum albumin (rHSA). This was based on pseudo-steady state mass balance, where the volumetric O-2 transfer coefficient, k (L) a, was estimated as a function of power input per unit volume and aeration rate. Under pseudo-steady state, the O-2 transfer rate model matched the O-2 uptake rate obtained from a previous macrokinetic model. This procedure was also applied to estimate biomass concentration by using the on-line rolling identification approach.