Ab initio RHF and MP2 calculations with the standard 6-31G* and 6-311G** basis sets were carried out on cyanomethyl formate (HCOOCH2CN). The calculations indicated two stable conformers (Z,ap and Z,sc) for this molecule. However, the energy difference between the conformers varied at different levels of theory. The calculations with the 6-311G** basis set and correlated wave functions indicated equal energy for these conformers, whereas all the other calculations slightly preferred the Zap conformer (by 0.2-2.1 kJ mol(-1)). The existence of the Z,ap and Z,sc conformers of cyanomethyl formate was confirmed by matrix isolation infrared studies in argon. Immediately after deposition the spectrum showed two peaks both in carbonyl and asymmetric C-O-C stretching regions. The behavior of these pairs of infrared peaks upon UV irradiation and the comparison with the results obtained for related esters strongly indicate that these doublets are caused by two different conformers. It was observed that the Z,ap and Z,sc conformers can be surmounted thermally in an argon matrix. This is consistent with the calculated values of 2.6 kJ mol(-1) (MP2/6-31G*) or 3.7 kJ mol(-1) (RHF/6-31G*) for the potential energy barrier of this process. A conformer interconversion process, proceeding from both the Z,ap and Z,sc to the E,sc conformer, was induced by UV irradiation at 248 nm The E,se conformer is the main product of isomerization. This was established by ab initio calculations and by analysis of the spectral changes in the (C=O) and (C-O-C) stretching legions. Upon prolonged irradiation at the same wavelength, the E,se conformer decomposed to stable products including a hydrogen-bonded (1:1) complex of CO and cyanomethanol. (CO2 production was also observed. However, its counterpart could not be identified, presumably due to further decomposition.)