The conformers of chloroformylketene Cl-CO-CH=C=O were generated by thermolysis of malonyl dichloride CH2(COCl)(2) at 50 degrees C and isolated in rare gas matrices. Both conformers were characterized by low-temperature infrared spectroscopy in argon and xenon matrices. The observed frequencies and intensities are in good agreement with the MP2/D95* ab initio calculations. The experimental spectra show that the s-trans conformer is more stable than the s-cis one. Consistent with this experimental result, high-level ab initio calculations predict that the two conformers have nearly identical energies. UV irradiations of chloroformylketene at lambda greater than or equal to 310 nm in rare gas matrices show interconversion of the conformers in the first step. In the second one, the s-cis conformer loses HCl to yield carbon suboxide. The reaction products (HCl and C3O2) are trapped in the same cage and form 1:1 complex, characterized by its stretching vibrational bands (2719.5 and 2232 cm(-1) in argon). Modeling of the photochemical processes, using ab initio calculations at the MP2/3-21G* level, is performed in the ground state (S-0) and the first singlet excited state (S-1). The isomerization barrier in the S-0 state (12 kcal mol(-1)) is similar to the data obtained for other acylketenes. This barrier is lower in the S-1 state (5 kcal . mol(-1)). The barrier for the reaction is higher in the SI state (53.8 kcal . mol(-1)) than in S-0 (30 kcal . mol(-1)), these theoretical results suggesting the reaction to take place in the ground state.