AbstractMeasurements ofkLa were carried out in 1. 5‐ and 5‐L New Brunswick Scientific CelliGen® bioreactors. The measuredkLa in water were identical for both vessel sizes operated in similar condition. The mass transfer rate increased with temperature, mixing speed, and aeration rate, with this last parameter being the most significant. Surface aeration alone gavekLa values of 0. 4 to 1. 6 h−1. A 25% decrease inkLa was observed above an aeration rate of 1. 6 vvm. This was caused by the particular foam breaker of the CelliGen bioreactor. Measurements ofkLa using a mammalian cell culture medium supplemented with 5% fetal calf serum (FCS) have confirmed the negative effect of the foam breaker onkLaThe measured value in this medium was 1. 2 h−1for all aeration rates, more than 60% of which was attributed to surface