The chloride ion affinities of SiCl2 and SiClF have been determined from threshold energy collision-induced dissociation in a flowing after glow-triple quadrupole instrument. Values of 244.8 +/- 13.9 and 205.4 +/- 8.5 kJ/mol were obtained for the chloride ion affinities of SiCl2 and SiClF. The measured chloride ion affinities can be used in thermochemical cycles with calculated data to give heats of formation of the halosilylenes SiCl2 and SiClF of -146.8 +/- 20 and -385.0 +/- 20 kJ/mol. G2 predictions for the heats of formation of the two silylenes are in good agreement with those derived from our experimental measurements. (C) 2004 Elsevier B.V. All rights reserved.