The study is focused on modeling of gas and liquid residence time distribution in an aerated liquid system of an inverse fluidized bed bioreactor. Two opposite strategies are currently available: the use of powerful complex Computational fluid dynamics (CFD) simulation and the phenomenological semi-empirical models. In this work, a specific methodology is proposed, as follows: the reactor is modeled as a reactor network containing a combination of zones with basic ideal flow patterns such as perfect mixed flow (PMF) and Plug flow (PF). The approach is based on a Mellin-modification of the Laplace transformation over the relevant equations. The method allows zero-time solutions for identification analysis. The study shows that the increase of the gas flowrate leads to higher mixing intensity of the gas phase. Decreasing the gas velocity, the inverse fluidized bed tends to perform as a plug flow reactor. The liquid phase performs closer to disperse plug flow. (C) 2008 Elsevier B.V. All rights reserved.