The infrared spectra of dichloromethylmethyldichlorosilane (Cl2CHCH3SiCl2) were recorded in the vapour, amorphous and partly crystalline phases and when isolated in argon, nitrogen and krypton matrices at 5 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 147 K. Spectra of the amorphous and annealed crystal deposited on a copper finger at 80 K were obtained. The spectra showed the existence of two conformers, anti and gauche, in the vapour and in the liquid. Approximately 12 infrared bands were reduced in intensity, and six of the same bands present in the fluid phases in the Raman spectra vanished upon crystallization. From the intensity variations of two band pairs with temperature, a ΔH° value of 0.4 ± 0.3 kJ mol−1 between the conformers was obtained in the liquid, anti being the low-energy conformer. Small increases and decreases in the IR band intensities were observed in the matrix spectra after annealing to 32 K (nitrogen), 36 K (argon) and 50 K (krypton). The enthalpy difference between the conformers is low in the matrices, but the anti conformer had lower energy than the gauche conformer. Ab initio calculations were performed at the HF/6–311G* level and gave optimized geometries, vibrational wavenumbers and infrared and Raman intensities for the anti and gauche conformers. The conformational energy derived was 4.4 kJ mol−1 with anti being the low-energy conformer. The dipole moments were calculated to be 1.2 and 3.1 D for the anti and gauche conformers, respectively. Correlation between the observed and calculated wavenumbers of both conformers revealed that anti was present in the crystal, and complete assignments of the spectra were carried out. Copyright © 2001 John Wiley & Sons, Ltd.