This project, part of the Experimental Physical Chemistry Program, is in the area of gas phase chemical kinetics. It involves the study of the reactions of polyatomic free radicals with halogen containing diatomic molecules, such as HI, HBr, and I2. The reactions will be studied using a heated tubular quartz reactor coupled to a photoionization mass spectrometer. Excimer laser photolysis will be employed to produce the polyatomic free radicals. This research will provide new knowledge of the dynamics of important elementary reactions and provide more accurate determinations of the thermodynamic properties, such as enthalpies of formation, of the free radicals studied. This information will be of great interest for the modeling of combustion processes, for the reliable prediction of the thermochemical properties of reactive species, and for determining plausible reaction mechanisms where knowledge of the potential energy along competing reaction paths is essential.