The photolysis (lambda greater than or equal to 230 nm) of 3,4-pentadienoyl chloride 1 isolated in argon matrix at 10 K as well as the follow-up reactions were studied by FTIR spectroscopy. The experimental data point out that the first step in the photochemical reaction is the elimination of HCl and the formation of an intermediate product identified as allenyl ketene 2. Further irradiation determines the decarbonylation of 2 leading to a very reactive carbenic species, allenylcarbene 3, which undergoes subsequent reactions. The final products identified were vinylacetylene 4 and acetylene 7. The assignment of the intermediates and final products as well as the reaction mechanism is supported by high-level theoretical calculations, LSDA/6-31G** and CASSCF/6-31G*.