Bond dissociation energies, D(Me3Si-X), and molecular heats of formation, H°f(Me3SiX)g, are derived from electron impact measurements on the trimethylsilyl halides (X = C1, Br and I). In particular D(Me3Si-C1) = 88 + 2 kcal mole-1, D(Me3Si-Br) = 78.5 + 2 kcal mole-1 and D(Me3Si-I) = 69 + 2 kcal mole-1. The method used for the measurement of appearance potentials is simple and rapid, utilising the high sensitivity of the MS9 mass spectrometer. The possibility of (p-d)II bonding in the trimethylsilyl halides is discussed. A brief description of exploratory work on the gas phase reactions between organosilanes and halogens is given and the system hexamethyldisilane, (Me3Si)2, + iodine discussed in detail. This system has been studied kinetically between I88 and 250°C at initial reactant pressures of 1.5 - 11.0 mm Hg. The simple stoiceiometry (Me3Si)2 + I2 = 2Me3 SiI exists under the conditions used and the reaction is 3/2 order overall, being first order in hexamethyldisilane and half order in iodine. The simple chain mechanism is suggested, leading to the overall rate equation where KI2 is the constant for the iodine equilibrium. The complex rate constants, K2KI21/2 are found to obey the Arrhenius equation the range studied allows the evaluation of k2, the rate constant for attack of iodine atoms on hexamethyl-disilane, at that temperature. In this way k2 is shown to be governed by the relationship: k2= 1.70 x 10 exp (-8100 +- 1,100)/RT with K2 in cc mole-1 sec-1 and energies in cal mole-1.