The molecular structures and conformations of methylvinyldifluorosilane (VFS), H2C=CHSiF2CH3, and methylvinyldichlorosilane (VCS), H2C=CHSiCl2CH3, have been studied by using gas phase electron diffraction (GED) data at 25 degrees C. Ab initio molecular orbital (MO) calculations were used to establish constraints in the theoretical model used to analyze the GED data. These molecules exist in the gas phase as a mixture of two conformers, syn (S, torsional angle phi(CCSiC) = 0 degrees) and gauche (G, torsional angle phi(CCSiC) close to 120 degrees). Relevant structural parameters for VFS (syn) are as follows : bond lengths (r(g)), r(Si-C-vinyl) 1.846(3) Angstrom, r(Si-CH3) = 1.851(3) Angstrom, r(C=C) 1.344(5) Angstrom, r(Si-F) = 1.592(2) Angstrom; bond angles (angle(alpha)), angle CSiC = 113.4-(11)degrees, angle CCSi = 123.3(8)degrees; torsional angle, phi(G) = 117(14)degrees. For VFS the experimental gas-phase composition (%) was (syn/gauche) 35/65 (+/-41)%. An estimated conformational energy difference Delta E degrees(G-S) = 0.0(+/-1.2) kcal/mol was obtained for VFS. Relevant structural parameters for VCS (syn) are as follows : bond lengths (r(g)), r(Si-C-vinyl) = 1.843(4) Angstrom, r(Si-CH3) = 1.855(4) Angstrom, r(Si-Cl) = 2.051(2) Angstrom, r(C=C) = 1.341(6) Angstrom; bond angles (angle(alpha)), angle CSiC = 111.6(17)degrees, angle CCSi = 123.3(17)degrees; torsional angle, phi(G) = 121(16)degrees. For VCS the experimental composition was (syn/gauche) 45/55 (+/-64)%. An estimated conformational energy difference Delta E degrees(G-S) = 0.3(+/-1.8) kcal/mol was obtained from this composition. Error estimates are given as 2 sigma (sigma includes estimates of uncertainties in voltage/height measurements and correlation in the experimental data).