Three very different techniques for measuring the catalyst wetted fraction - or wetting efficiency - in trickle bed reactors have been carried out and compared. The first one based on pressure drop data performed successively in single gas then liquid phases and, then, in a gas - liquid phase is experimentally very simple but did not result in meaningful wetting efficiency estimation. The other ones, based on RTD analysis and on dye adsorption on wetted surfaces, gave similar results and were used to investigate different parameters: liquid velocity and gas molecular weight and pressure by RTD, liquid - solid interaction or affinity (contact angle) and particle shape and diameter by dye fixation. The main operating parameter is the liquid velocity while the effect of gas flow is very weak. Higher solid - liquid affinity (heptane versus water) improves wetting efficiency only at very low liquid velocity (< 2 x 10(-1) m/s).