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The initial probabilities of precursor-mediated dissociative chemisorption of 13 C 2 H 6 , C 2 D 6 , C 3 H 8 , CH 3 CD 2 CH 3 and C 3 D 8 have been measured on the hexagonally close packed Ru(001) surface. The activation energies for CH bond cleavage are 9310 cal mol −1 for 13 C 2 H 6 and 9900 cal mol −1 for C 2 D 6 with respect to the bottom of the physically adsorbed well of the ethane molecule. For each isotopomer of propane, these measured activation energies are 10 430, 10 620 and 10 910 kcal mol −1 for C 3 H 8 , CH 3 CD 2 CH 3 and C 3 D 8 , respectively. For each alkane investigated the ratio of the preexponential factor for desorption relative to that of reaction is ∼ 100 due to the larger phase space available to the molecule for desorption relative to reaction. An examination of the selective activation of primary (1°) versus secondary (2°) CH bonds in propane reveals that 2° CH bond cleavage is favored energetically by 240 cal mol −1 and entropically by a factor of 2.1 over 1° bond cleavage. However, the formation of 1° Ru-propyl intermediates is favored over the formation of 2° Ru-propyl intermediates at all investigated temperatures due to the greater number of 1° CH bonds in propane compared to 2° CH bonds. The measured differences in CH versus CD bond activation for both ethane and propane are attributed to zero-point energy differences between each isotopomer and point to classical over the barrier reaction dynamics as the reaction pathway.