The adsorption of gaseous H and CH3 species to H-terminated beta-SiC clusters of different sizes and shapes have been investigated theoretically using ab initio molecular orbital theory, including the effects of electron correlation by means of second-order Moller-Plesset (MP2) perturbation theory. Corrections for basis set superposition errors (BSSE) were also performed. An obvious dependence of cluster shape on the adsorption energy was found. The order of energies for H adsorption was found to be cluster I (478) > cluster II (398) cluster III,a (365) approximate to cluster III,b (366). (The unit of energies presented here is kJ/mol.) Cluster 1 and IV represent a smaller and a larger roundish cluster, while cluster II represents an (111) surface and cluster III (a,b) different sites of an (110) surface. The order of energies was somewhat different for CH3 adsorption: cluster I (521) > cluster III,b (421) > cluster II (346) > cluster III,a (269). As a comparison, adsorption of F/CF3 to corresponding F-terminated clusters was also studied at various levels of theory. The adsorption energies for the smallest cluster I were found to be 621 kJ/mol for F and 297 kJ/mol for CS at the MP2/BSSE level of theory. More generally, a tendency was observed where the gaseous F was found to bind more strongly to the clusters than the H species does, while the gaseous CF3 was found to bind more weakly to the clusters than the CH3 species does (at the HF level of theory). Furthermore, bond strength was also influenced by cluster size. The H species was found to bind more strongly to the smaller roundish cluster I (484) than to the larger roundish cluster IV (369) (at the MP2 level of theory).