Fermentation experiments using Aspergillus niger result in a very viscous broth due to the growth of filamentous microorganism. For viscous fermentation processes, it is difficult to estimate with confidence the volumetric oxygen mass transfer coefficient ( KLa), which can be used for scale-up or design of bioreactors. In the present study, four methods based on dynamic and stationary approaches were used to measure KLa throughout the fermentation. Data reconciliation was used to obtain a more reliable and consistent KLa. The KLa value obtained by a data reconciliation technique was found to be more reliable since it takes into consideration both the reliability of all measured variables and the accuracy of all mass balance equations.