There are many dynamic methods for measuring the volumetric mass transfer coefficient. The “gas out–gas in” method can directly determine the volumetric mass transfer coefficient in a bioreactor system and provide estimates of the volumetric microbial oxygen uptake rate and the average oxygen saturation concentration at the gas–liquid interface. The errors on these parameters are large if the dissolved oxygen probe response time is not considered. For reliable measurements, deconvolution of the oxygen probe measurements must be made. © 1995 John Wiley & Sons, Inc.