Improving the selectivity of the transformation of heavy oils into valuable middle distillates represents one of the current challenges for hydrocracking (HCK) catalysts. For that purpose, new insights are required for a better design of HCK catalysts. We choose the hydrocracking of phenanthrene as a model reaction in order to compare several mesoporous and microporous acid solids. Characterization of the acidity of the solids reveals that there is no direct correlation between the HCK selectivity and the solids acidity. Moreover, combining the results of catalytic tests and force-field simulations, we are able to show that diffusional limitations can be ruled out and that the key phenomenon adversely affecting the selectivity is a too strong adsorption of isomerized products which favors overcracking. Below a steric exclusion threshold, pore size controls energies by virtue of confinement effects. Optimal solid acid HCK catalysts should therefore be designed by taking those effects into account. (C) 2003 Elsevier Science (USA). All rights reserved.