The relationship between the geometry and the operating conditions, hydrodynamics and performance of an industrial gas-liquid stirred reactor has been studied with the help of CFD modeling and gamma ray tomography. It is seen that the interfacial area distribution is very wide. This in some areas leads to mass transfer limitations. Strategies for retrofitting the reactor were then developed (e.g. change in the operating speed, change of impeller type, change in the feed introduction, etc.) and tested by CFD for improving the reactor performance. The benefits of the implementation were in terms of the improvement in the product quality, reduction in the by-product formation, increase in the reactor throughput. (c) 2005 Elsevier Ltd. All rights reserved.