Based on a comparison between measured and simulated adsorption properties, we demonstrate that a decrease in the Gibbs free energy of formation and adsorption-due to higher adsorption entropy-satisfactorily explains the selective production and adsorption of the most compact, branched paraffins in n-hexadecane hydroconversion in molecular sieves with pore diameters of similar to0.75 nm. Adsorption entropy is important because the pores are saturated with reactant, and because the adsorbed phase is not at gas-phase chemical equilibrium. This explanation supplants the traditional kinetic explanation involving changes in the Gibbs free energy of formation of the relevant transition states. Instead, we attribute the effect of molecular sieve structure on the branched paraffin yield to a redirection of the hydroisomerization reactions away from the gas-phase chemical equilibrium distribution, commensurate with the Gibbs free energy of adsorption of the isomers inside the pores. These shape-selective changes to the reaction rates appear to be as ubiquitous as those originating from steric constraints imposed on intracrystalline diffusion and reaction rates. This would make adsorption-induced changes in the Gibbs free energy of formation of reactants, intermediates, and products a missing cornerstone in traditional shape selectivity theory. (C) 2003 Elsevier Science (USA). All rights reserved.