A double-slit model developed can predict the frictional two-phase pressure drop, external liquid holdup, pellet-scale external wetting efficiency, and gas–liquid interfacial area in cocurrent downflow trickle-bed reactors operated under partially wetted conditions in the trickle flow regime. The model, an extension of the Holub et al. (1992, 1993) mechanistic pore-scale phenomenological approach, was designed to mimic the actual bed void by two inclined and interconnected slits: wet and dry slit. The external wetting efficiency is linked to both the pressure drop and external liquid holdup. The model also predicts gas–liquid interfacial areas in partially wetted conditions. An extensive trickle-flow regime database including over 1,200 measurements of two-phase pressure drop, liquid holdup, gas–liquid interfacial area and wetting efficiency, published in 1974–1998 on the partial-wetted conditions, was used to validate the modeling approach. Two new improved slip-factor functions were also developed using dimensional analysis and artificial neural networks. High-pressure and -temperature wetting efficiency, liquid holdup, pressure drop, and gas–liquid interfacial area data from the literature on the trickle-flow regime using conventional monosized beds and catalyst bed-dilution conditions were successfully forecasted by the model.