This work deals with mathematical modeling of the fermentation process performed in an internal loop airlift reactor (ILALR) using a tank in series model. Fermentation of gluconic acid by the strain Aspergillus niger was chosen as a reaction system. Fermentations were carried out in three laboratory IALRs of different scale (12, 40 and 200 dm(3)). Data from the smallest reactor were used for kinetic parameters estimation. All other model parameters, i.e., the gas hold-ups, the circulation velocities of the liquid and the gas, the volumetric mass transfer coefficient of oxygen were predicted by employing experimentally determined correlations from the literature, verified in our reactors using the model system water-air. The results of simulations and experiments are in a good agreement. (c) 2007 Elsevier Ltd. All rights reserved.