The hydrogenation of propyne over an ion-exchanged copper on silica catalyst has been investigated. The system has a high selectivity for the production of propene over propane but is accompanied by the formation of oligomeric material that causes catalyst deactivation. Oligomer deposition can be readily followed by in situ infrared spectroscopic measurements. The amount of oligomer falls with increase in temperature and hydrogen pressure. The mechanism has been investigated through analysis of the oligomer fraction and by the use of deuterium in place of hydrogen. Greater than 70% of the C6 products are dienes, principally trans-2-methylpenta-1,3-diene, trans-2-trans-4-hexadiene, and 2,3-dimethylbuta-1,3-diene. Three C6H8 compounds, 2-methylpent-1-ene-3-yne and the cis and trans isomers of hex-2-ene-4-yne make up the bulk of the remainder. Their amounts decrease steeply with increase in hydrogen pressure. The reaction with deuterium leads to dienes with an average of two deuteriums and ene-ynes averaging almost one deuterium. Unreacted propyne is exchanged to a significant extent very largely at the C1 position. Product propenes contain zero to three deuterium atoms distributed throughout the vinyl part of the molecule. There is a moderate kinetic isotope effect on going from hydrogen to deuterium which affects propene production to a much greater extent than oligomer formation. The significance of these findings in relation to the steps by which hydrogenation and oligomerisation occur is discussed.