Mixing determines the micro-environment for most (bio)chemical conversions. Estimation of mixing behaviour is therefore of major importance for the design and study of stirred reactors. To predict the mixing behaviour of stirred (bio)reactors with multiple impellers, commonly used in industry, a multi-compartment mixing model was developed. Experimental data on mixing behaviour were obtained with a fluorescent tracer pulse response technique. Various pulse-response curves have been measured in a large scale bioreactor (30 m(3)) with different geometries. Working volume, number of impellers, clearance between impellers and operating conditions were changed. Comparison between the simulated data and the experimental data shows that the developed model predicts the measured pulse-response curves rather well. The model parameters are obtained from available knowledge on hydrodynamics instead of fitting experimental data. This will increase the applicability of the model.