The conversion of 3-methylpentane (3MP), 2-methylpentane (2MP), n-hexane (nH), and methylcyclopentane (MCP) was investigated on 0.3 and 10% Rh/Al2O3 and 5% Rh/SiO2 as a function of hydrogen pressure and temperature. The catalysts were prepared by the incipient wetness method. Metal accessibility was 57, 18 and 36%, which corresponded to mean particle size of 1.5, 5 and 2.5 nm, respectively. The hydrogenolytic cleavage of hydrocarbon C-C bonds was the main reaction. Skeletal isomers (up to 20%) were formed from methylpentanes on 10% Rh/Al2O3. C-5-cyclization was a minor reaction (less than 10%) and was promoted by low p(H-2). Particle size effect was clearly observed in the non-degradative reaction path; this route was favored by larger Rh particles. Single splitting of C-C bonds was catalyzed at high hydrogen coverage. Decreasing P(H-2) caused "deepening" of the hydrogenolysis and the catalysts lost much of their activity. The hindrance in the re-hydrogenation of the surface intermediate of fragmentation was proposed to explain the positive hydrogen order. The role of further hydrogenolysis of particular fragments or ring opening intermediates was significant at low p(H-2). Reaction conditions governed the desorption or the further reactions of the surface intermediates. For instance, fragments were produced in the conversion of MCP mainly from branched ring opening intermediates. The fragmentation patterns of hexane isomers were successfully applied for modeling the fragment distribution of MCP. (C) 2002 Elsevier Science B.V. All rights reserved.