We have employed molecular beam techniques to investigate the initial probability of direct dissociative chemisorption of perhydrido- and perdeuteromethane, ethane, propane,,and cyclopropane on Ir(110) as a function of beam translational energy from 1.5 to 59 kcal/mol. For each of the saturated perhydridoisotopomers, a measurable (>0.02) initial probability of direct dissociative chemisorption is observed above a beam energy of approximately 7 kcal/mol. in the case of the saturated perdeuteroisotopomers, this "threshold" energy is approximately 10 kcal/mol. For cyclopropane, the relevant energy is approximately 5 kcal/mol for both c-C3H6 and c-C3D6. Above these energies, the initial probability of direct chemisorption of each of these hydrocarbons increases approximately linearly with beam translational energy and is independent of surface temperature, indicative of direct dissociative chemisorption. However, the energy required to achieve a measurable (>0.02 in this experiment) initial probability of direct chemisorption is independent of carbon chain length for the saturated perhydrido- and perdeuterohydrocarbons, and is slightly lower for cyclopropane. This observation is explained by considering bond energetics, since in each case the initial bond cleaved (C-H for methane, ethane, and propane, and C-C for cyclopropane) upon chemisorption is of comparable strength.