Molecular competition effects in the hydroconversion of an equimolar heptane/nonane mixture were studied in liquid-phase reaction conditions in a fixed-bed reactor filled with a Pt-ultrastable-Y (USY) catalyst. Liquid-phase conditions were attained by feeding the hydrocarbon mixture with hydrogen dissolved in it at 100 bar and 230degreesC. Comparative vapor-phase experiments were run at 230degreesC, a pressure of 4.5 bar, and a hydrogen-to-hydrocarbon ratio of 13. Whereas catalytic experiments in the vapor-phase showed a marked referential conversion of nonane over heptane, in liquid-phase the differences in conversion rate between nonane and heptane were much less pronounced. Adsorption-reaction models were used to explain the difference. For this purpose, intrinsic kinetic constants for heptane and nonane were derived from experimental data from vapor-phase conversions of the n-alkanes individually, using an adsorption-reaction model with independently determined adsorption equilibria, and assuming the classic bifunctional reaction scheme. The liquid-phase conversion of the heptane/nonane mixture was predicted very well using these intrinsic reaction kinetics derived from the vapor-phase experiments and assuming no adsorption preference between heptane and nonane. In contrast to this, the conversion of the heptane/nonane mixture in the vapor phase could only be appropriately described by a model involving adsorption according to a Langmuir-with-interaction model, favoring adsorption of the heaviest compound. In liquid-phase reaction conditions and at saturation of the Pt-USY zeolite pores with n-alkanes, there is no such selective adsorption of the heaviest compound. In liquid-phase, the conversion of the mixture reflects the intrinsic reaction kinetics of the individual compounds.