Abstract Phase transition temperatures of capillary condensed organic liquids in porous silica were measured using a 1020/DSC system (Perkin-Elmer Ltd.). The amounts and types of hydroxyl groups on the sample surfaces were precisely controlled to discuss the surface effect on the phase transition temperature. The melting point of the solid phase of benzene in the pores decreased with increasing concentration of the surface hydroxyl groups. On the other hand, in the case of n-hexane, such an effect was not observed. From these results, it is assumed that the liquid structure of capillary condensed benzene in the pores changes with the amounts and types of surface hydroxyl groups. This consideration is confirmed from IR spectroscopy.