Gaseous reaction equilibria in the Si-O-I system were investigated by mass spectrometry, leading to identification and thermochemical characterization of the species OSiI, OSil(2), Sil, and SiI2. Species were generated by the addition of gaseous I-2 to an effusion cell reactor containing a Si/SiO2 mixture of composition "SiO". Equilibria involving the Si-O-I species were measured in the range 1350 to 1540 K. From third law analysis, the enthalpies of formation of OSLI, OSiI2, Sil, and SiI2 were derived as greater than or equal to -21.7 +/- 5, greater than or equal to 74.5 +/- 5, 61.0 +/- 2, and 21.6 +/-3.3 kcal mol(-1), respectively. Although the values for SiI and SiI2 are in good agreement with selected values in a recent compilation, the results for OSLI and OSiI2 are 25-30 kcal mol(-1) less stable than semiempirical estimates developed for modeling of metal halide lamp chemistry. In some instances, other estimated thermochemical database values differ substantially from both theoretical and experimental results; reasons for these discrepancies are discussed.