The gas-phase structures of tert-butyl vinyl ether, t-BuOC(H)=CH2 (t-BVE), and (Z)-methyl 1-propenyl ether, MedC(H)=C(H)Me (Z-MPE), have been studied by gas electron diffraction and ab initio calculations (HF/3-21G and MP2/6-31G*). For both compounds only a single conformer with a slightly nonplanar anti structure of the C=C-O-C skeleton was observed. The vibrationally averaged dihedral angles phi(C=C-O-C) are 167(5)degrees (t-BVE) and 161(5)degrees (Z-MPE), respectively. The experimental scattering intensities are fitted slightly better with dynamic models and double minimum potentials for the internal rotation around the O-C(sp(2)) bonds. The dihedral angles of the equilibrium structures phi(e)(C=C-O-C) are 170(5)degrees for t-BVE and 161(8)degrees for Z-MPE. The potential barriers at the exactly planar anti conformation (phi(C=C-O-C)=180 degrees) are V-0 = 0.15(10) and 0.20(10) kcal mol(-1), respectively. The experimental results are reproduced very well by MP2/6-31G* calculations (phi(e) = 169.0 degrees and 154.2 degrees and V-0 = 0.12 and 0.11 kcal mol(-1) for t-BVE and Z-MPE, respectively). The HF/3-21G method, however, predicts a planar structure (phi(e) = 180 degrees) for t-BVE and an anticlinal structure (phi(e) = 138.7 degrees) for Z-MPE.