The heats of formation, Delta H-f 298, of vinyl and formyl halides calculated using G2 theory are -145.9 (CH2-CHF), 21.8 (CH2CHCl), 79.6 (CH2CHBr), 147.1 (CH2CHI), -343.7 (HF-CO), -192.7 (HClCO), -133.9 (HBrCO), and -66.2 kJ mol(-1) (HICO), which agree with the available experimental data. While the covalent radii of sp(3) and sp(2) hybridized carbons are 0.762 and 0.735 Angstrom, respectively, at the MP2/6-31G(d) level, the C-X bond lengths in formyl bromide and iodide are 0.016 and 0.045 Angstrom, respectively, longer when compared with those in methyl bromide and iodide. However, the C-F bond in formyl fluoride is 0.040 Angstrom shorter than the C-F bond in methyl fluoride. The C-X bond dissociation energies (DC-X) for vinyl, formyl, and methyl halides show good linear correlations with the Mulliken and Pauling electronegativites of the halogens. The DC-X values for formyl bromide and iodide are smaller than those calculated for methyl bromide and iodide. The stabilization energies (SE) yielding estimates of the stabilization of the C=C and C=O double bonds by halogens decrease from X = F to X = I and show reasonable linear correlations with the Mulliken and Pauling electronegativities of the halogens. While the Dc-x energies for formyl halides are smaller than those for vinyl halides, the SE(formyl) values are larger than the corresponding SE(vinyl). The elimination of HX from formyl halides is exothermic, in contrast to the endothermic elimination of HX from vinyl halides.