Gas-phase reactions between cyclohexane and Re/sub n/(CO)/sub m//sup +/ (n = 3, m = 7-13 and n = 4, m = 8-16) were characterized by ion cyclotron resonance techniques. The reactant cluster ions were formed by reaction between Re/sub 2/(CO)/sub 10/ and its electron impact produced fragments. Reactions of the clusters with cyclohexane produced species of the type Re/sub n/C/sub 6/H/sub 2(6-x)/(CO)/sub (m-y)//sup +/ corresponding to loss of x molecules of H/sub 2/ and y molecules of CO from a complex of the reactants. Reactions were observed for only the most coordinatively unsaturated clusters (m < 10 for n = 3 and m < 12 for n = 4). H/sub 2/ yield also tended to increase with coordinative unsaturation. The results are discussed in terms of a mechanism involving oxidative addition of a C-H bond to a cluster metal atom