Comparison of different experimental techniques has shown that the best evaluation of the wetted areas of a catalyst is obtainable by a tracer method The static areas may be evaluated from the static hold-up values as determlned by fitting the liquid RTDF with the PE model. The importance of the "q~lality" of wetting on the conversion rate in trickle-bed reactors is also dlscussed. INTRODUCTION Partial external wetting of catalyst pellets in trickle-bed reactors affects the conversoon yield and the reactor stability (Sedriks and henney, 1973, Germaln, Lefebvre and L'Homme, 1974, Hamka and co-workers', 1976, Germaln and co-workers', 1978). For non--volatile liquid reactants, m fact, the wetted areas 1s one of the parameters affecting the rate of their supply to the solid catalyst, If the reactlon rate is very fast, it may even be the controlling parameter. Moreover, If reactants are present m the gas phase, they can be supplied to the catalyst much faster If partly wetted (Germam and co-workers', 1978). Some models have been proposed to take account of the influence of partial wetting on reactor performance (Mears, 1974, Duducovic@, 1977, Morita and Smith, 1978, Germain and co-workers', 1978). The very few data on the wetted fraction, however, have been obtained with different experimental methods and do not agree. As the liquid flow rate increases, some results show a constant wetted area (Schwartz, Weger and Du-This may support the hypothesis that d/(D,)app/Di actually represents the wetted area fraction a, If this 1s true, aC virtually equals the dynamic area ad.