The geometric structures and conformational properties of 1,1-dimethoxyethene (1,1-DME), (Z)-1,2-dimethoxyethene (Z-1,2-DME), and tetramethoxyethene (TME) have been determined by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G, HF/6-31G*, and MP2/6-31G*). Additional theoretical calculations have been performed for (E)-1,2-dimethoxyethene and trimethoxyethene. The calculations : predict three or more-possible conformations for these compounds in the energy range below about 2 kcal mol(-1). For 1,1-DME, the GED experiment results in a mixture of two conformers. The main form (61(7)%) possesses C-2v symmetry, with both methoxy groups synperiplanar to the C=C double bond (phi(1,2)(C=C-O-C) = 0 degrees). In,the second conformer,lone methoxy group is oriented synperiplanar and the other one anticlinal (phi(2) = 131(7)degrees). For Z-1,2-DME and TME, only one conformation was observed in the GED analyses. Z-1,2-DME possesses a structure with syn- and antiperiplanar (phi(2) 152(6)degrees) methoxy groups (C-1 symmetry). In TME, all groups are oriented anticlinal, alternatingly above-below-above-below the molecular plane (D-2 symmetry). This compound is twisted around the C=C bond by 14(3)degrees. The MP2 calculations reproduce the conformational properties of these methoxyethenes perfectly, whereas the HF/3-21G approximation leads to incorrect results. The Mp2/6-31G* method predicts for the preferred conformation of (E)-1,2-dimethoxyethene a structure With C-2h symmetry and both methoxy groups synperiplanar to the C=C bond. For trimethoxyethene, eight minima were detected on the energy surface. In the ground-state structure, two groups are oriented anticlinal and one synperiplanar.