The infrared (IR) spectra of chloromethylmethyl dichlorosilane (ClCH2CH3SiCl2) were recorded in the vapour, amorphous and the crystalline solid phases in the range 4000–50 cm−1. The middle IR spectra of the compound, isolated at 4.8 K in the argon and the nitrogen matrices were also observed. Raman spectra of the liquid were recorded at various temperatures between 298 and 218 K in a capillary. The crystal was studied in a capillary at 210 K and additional spectra of the amorphous and annealed crystal, which was deposited on a copper finger cooled with liquid nitrogen, were recorded.The spectra of chloromethylmethyl dichlorosilane showed that two conformers — anti and gauche — were present in the vapour and in the liquid. Approximately five Raman and three IR bands, which were all weak, were present in the fluid phases: these vanished upon crystallization. Three band pairs were used to calculate the enthalpy difference between the conformers in the liquid phase and this gave the average following enthalpy difference: ΔH(liq)(anti–gauche) of 0.2±0.3 kJ mol−1 with gauche being slightly lower in energy. In the argon and nitrogen matrices, small intensity variations in the IR bands were observed after annealing to ca. 36 K (argon) or 32 K (nitrogen). This implies that there is a low enthalpy difference between the conformers in the matrices, again gauche has the lower energy.The optimized geometries, IR and Raman intensities, and scaled vibrational frequencies for the anti and gauche conformers were derived from ab initio calculations using the Gaussian 94 program with a HF/6-311G∗ basis function. The derived conformational energy difference was 7.2 kJ mol−1, gauche being the conformer of lowest energy. The anti and gauche conformers had calculated dipole moments of 4.2 and 2.0 debye, respectively. Correlation between the observed and calculated wavenumbers of both conformers revealed that gauche was present in the crystal, and complete assignments of the spectra have been carried out.