Unique gas and liquid phase radioactive tracer experiments have been performed in horizontal cross-flow bubble column reactors of a commercial ethylbenzene oxidation plant. These experiments validated CFD modelling predictions, which in turn formed the basis for evaluation and selection of potential measures to reduce "oxygen starvation" predicted to occur for certain configurations and operating conditions. After implementation in commercial reactors lower operating temperatures, improved selectivity and thus better plant yields were achieved. (c) 2007 Shell Global Solutions International B.V. Published by Elsevier Ltd. All rights reserved.