A focus of this work has been to assess the range of applicability of a microcanonical unimolecular rate theory (MURT) model for activated dissociative chemisorption and to extract reactive transition state characteristics from analysis of varied thermal equilibrium, nonequilibrium, and quantum-state resolved experiments. Earlier work had concentrated on the analysis and simulation of CH₄ and SiH₄ dissociative chemisorption on several surfaces because this reactive step can limit the rates of natural gas reforming and the chemical vapor deposition of silicon. This year, we demonstrated that (i) the dissociative chemisorption and associative desorption dynamics of H₂ on Cu(111) and (ii) the CO₂ dissociative chemisorption and CO oxidation dynamics on Rh(111) can be modeled well with the MURT. For both systems, treating rotation as a spectator to the reaction dynamics was a good approximation at thermally accessible energies. The surface plays a crucial role in the activated dissociative chemisorption of H₂ and CO₂ under thermal equilibrium conditions where phonons are calculated to contribute 30-40% of the energy required to surmount the activation barrier for dissociation. MURT analysis of experiments indicates that the threshold energy for activated dissociative chemisorption of H₂ on Cu(111) is E₀ = 62 kJ/mol, whereas E₀ = 73 kJ/mol for CO₂ on Rh(111). Analysis of the CO₂ product angular yields and translational and rovibrational energy distributions from CO oxidation on Rh(111) indicate that an indirect reaction mechanism dominates under reactive molecular beam conditions. It is likely that a thermalized intermediate involving chemisorbed O (e.g., a carbonate species) facilitates the indirect CO oxidation pathway. Moving to larger molecules, current work concentrates on measuring dissociative sticking coefficients, S(T_g, T_s), for alkanes on Pt(111) as a function of the impinging gas temperature, T_g, and surface temperature, T_s, using an effusive molecular beam. Methane, ethane, and propane measurements have been made to date and higher alkanes through neopentane will be investigated on Pt(111) and other surfaces. An interest is to find at what level of alkane complexity the simple MURT model of dissociative chemisorption will break down. As the lifetime of the gas-surface collision complexes increases with molecular size, will energy transfer (e.g., complete thermalization of a precursor) and/or migration to step edge sites become critical new issues to address in modeling the overall gas-surface reactivity? Chemical trends will be investigated.