Photodissociation of propyne at 157 nm has been investigated using photofragment translational spectroscopy. Detailed investigation of various photofragments from the deuterated compounds CD3CCH and CH3CCD, as well as the unlabeled propyne provides a uniquely clear picture of an inherently complex process. Hydrogen atom elimination processes from both the CH3 group and the C=C-H group have been clearly observed. H atom elimination from the methyl group appears to be a single dynamical process, while ethynyl H elimination shows two distinctive dynamical pathways with a ratio of 0.30 (fast): 0.43 (slow). The relative contribution of the atomic hydrogen elimination from the two different sites was determined to be 0.73 (ethynyl): 0.27 (methyl). Molecular hydrogen elimination processes have also been observed, but with a much smaller yield compared to the atomic hydrogen elimination (1:9.6). Comparison of the H-2 HD and D-2 photoproducts from various deuterated propyne molecules shows that the molecular hydrogen elimination process is not sensitive to the origin of the two hydrogen atoms. This implies that scrambling (or isomerization) of H atoms is important prior to dissociation at 157 nm excitation of propyne. Two different C-C bond breaking processes have also been observed; one process breaks the C-C single bond to form methyl and C2H radicals, while the other process forms CH2 and C2H2. The existence of the CH2 channel also indicates that isomerization of propyne is significant prior to dissociation. The relative branching ratio of these two channels is estimated to be 2.2:1 for CH3 and CH2 formation, respectively. (C) 2000 American Institute of Physics. [S0021-9606(00)00913-2].