The hydrogenation of the main components of an anthracene oil has been investigated in a cocurrent downflow trickle bed reactor at 623 K and 9.8 MPa. The catalyst employed was sulfided Ni-Mo/gamma-Al2O3. The product of the effectiveness factor and chemical rate constant was determined for the reactions involved in the hydrogenation of naphthalene, acenaphthene, phenanthrene, anthracene, fluoranthene and pyrene. Reaction rates were expressed as a weighted average of the rate on the liquid-covered surface and on the dry surface. The external surface of the catalyst was considered to consist of a dynamic and a low liquid renewal area. The latter, identified as the area wetted by stagnant liquid, was assumed to be not reactive. The product of the effectiveness factor and the intrinsic rate constant were determined for dry and wetted catalyst surfaces by employing an estimation of the unit fraction of inactive surface. An estimation of the effectiveness factors was performed by employing the kinetic constants previously determined by Rosal et al. (Ind. Eng. Chem. Res., 31 (1992) 1007-12). The results showed that the effectiveness factor for the wetted surface followed a pattern in good agreement with theoretical predictions and was especially low for the hydrogenation of anthracene, a reaction whose rate was relatively high.