A model for external wetting efficiency of the catalyst in trickle-bed reactors was developed based on 1-D force-balance equations incorporating drag forces in a three-phase system. The gas-liquid interfacial drag was considered, as well as the tortuosity effect. The overall model was tested with major experimental data of a 15.2-cm-dia. trickle-bed reactor using a radioisotope tracer technique. Five catalyst packings, different in shape and size, were used in the experiment. External wetting efficiency was calculated from the square root of the ratio of the effective diffusivities in two-phase and liquid-filled operation. The model was also compared with other experimental data available in the literature. The predictions were within an average confidence limit Of +/- 15%.