The 0 K onsets (E-0) of a series of trichlorosilane derivatives SiCl3R -> SiCl3+ + R (R = Cl, H, CH3, C2H5, C2H3, CH2Cl, SiCl3) are measured by threshold photoelectron-phol(lion coincidence spectroscopy, Tile wellknown heat of formation Of SiCl4 is used as all anchor to determine the heat of formation of SiCl3+, which is, in turn, used as an anchor to determine the heats of formation of the other alkyltrichlorosilanes itivestiva ed. A series of isodesmic reactions at the G3 and CBS-QB3 levels are shown to accurately reproduce the experimental heats of formation, and this scheme is used to calculate the heat of formation Of Si2Cl6, from which the measured E-0 determines the SiCl3+ heat of formation. The measured values then determine the IF Of SiCl3+ along with the Si-R bond dissociation enthalpies of the six neutral species investigated. The experimental heats of formation are also used in a series of isodesmic reaction calculations to determine tile heats of formation of SiH3R (R = H, CH3, CH5, C2H3, CH,Cl, SiCl3).