A double-slit model de®eloped 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 condi-( tions 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 ®oid 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 exten-si®e trickle-flow regime database including o®er 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 ®alidate the modeling approach. Two new impro®ed slip-factor functions were also de®eloped 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 con®entional monosized beds and catalyst bed-dilution conditions were successfully forecasted by the model.