AbstractThe catalyst bed dilution technique has found rather widespread applications as a means of producing reliable and reproducible results for successful scale‐up and scale‐down when testing commercially used catalyst particle sizes and alternative feedstocks in small‐scale laboratory trickle‐bed reactors. In such small‐diameter laboratory units when the catalyst used in commercial units is employed as the only packing, low liquid velocities lead to incomplete catalyst utilization due to incomplete external catalyst wetting, liquid maldistribution, and wall effects. Diluting the bed with fines is recommended to remedy these shortcomings. The effect of the bed dilution with fines on catalyst wetting efficiency, pressure drop, and liquid holdup are quantified. At atmospheric pressure, fines improve the catalyst wetting efficiency by increasing both the pressure drop and liquid holdup. At high‐pressure operation, wetting efficiency improves further, whereas pressure drop increases and liquid holdup decreases. In a highly diluted bed (e.g., 2 or more volume fines/1 volume catalyst) the improvement in catalyst wetting efficiency is about the same as that achieved in a bed packed according to our recommended procedure, which is based on filling the voidage of the original bed with fines.