A new model predicting the conversion in a trickle-bed reactor is presented. The model partitions the interstitial volume, represented by a network of cells, into two parallel, well-mixed regions with different liquid velocities. The only model parameter is the standard deviation of the liquid velocity, which may be determined by a tracer experiment. The model accounts for the impact of velocity variation on liquid hold-up, wetting, and external mass transfer, and hence on the overall effectiveness factor. The isothermal decomposition of hydrogen peroxide on a NiO on gamma-alumina catalyst was used to test the model predictions of the dependence of conversion on space time for different bed lengths and temperatures. The corresponding Thiele moduli were in the range of 6.5-13. Space time and temperature were varied to obtain conversions in the range of 30-99%. In the pulsing flow regime, the conversion is close to that expected for a plug-flow reactor, whereas in the trickle-flow regime, the deviation of the conversion from that of the plug-flow model increases as the liquid velocity is decreased. The model predictions for high levels of conversion are more accurate than those of existing published models. This is probably due to its being the only one accounting for the impact of velocity dispersion on local variations in the overall effectiveness factor.