Gaseous mixtures of ketene with C2H4, and with C2H2, diluted in added argon to a total pressure of 400 Torr have been photodecomposed at room temperature by 308 nm u.v. radiation from a pulsed exciplex laser. Conversions were limited to ca. 2% and a reasonably complete hydrocarbon product analysis achieved (H2 was also monitored). In the C2H4 system products up to C5 were detected whilst in the C2H2 system products up to C6 were found. The product distributions suggest the intermediacy of ethyl and vinyl radicals (C2H4 system) and vinyl and propynyl (C3H3) radicals (C2H2 system) as well as the involvement of methylene (CH2, both 1A1 and 3B1 states), methyl, methyne and H atoms in both systems. Most of the products arising from the C3H3 radicals are reported for the first time in this system. Kinetic modelling based on the Gear algorithm has been used to predict the product distribution and its dependence on either C2H4 or C2H2 pressures. The modelling suggests the following. (a) The added gases scavenge CH2 predominantly, if not exclusively, in its 1A1 state with the following rate constants (in cm3 molecule–1 s–1)1CH2+ C2H4→ C3H6 2.1 × 10–10, 1CH2+ C2H2→ C3H4 2.5 × 10–10. (b) The CH2(3B1) analogues of these processes are probably not occurring and upper limits to the rate constants (same units) are 3CH2+ C2H4→ C3H6 –16, 3CH2+ C2H2→ C3H4 –16. (c) C3H3 radicals are formed via the decomposition of vibrationally excited C3H4, with a rate constant, k= 4.5 × 108 s–1, consistent with an R.R.K.M. theoretical estimate. (d) Most of the remaining products in both systems are formed via radical–radical reactions with plausibly high collision efficiencies. These findings are compared and contrasted with previous flash-photolysis and other studies of ketene photodecomposition.