Abstract Density Functional Theory calculations on E(SbMe2)2, E = O, S or Se, Me = CH3, indicate that the equilibrium structures are syn-syn or near syn-syn conformers with over-all C2 symmetry. The barriers restricting rotation about E-Sb bonds are very low, the equilibrium values for the dihedral angles ϕ(Sb-E-Sb-lp) where lp denotes the direction of the electron lone pair on one of the Sb atoms are probably less than 45°. The calculations further indicate the existence of syn-anti conformers some 4 kJ mol-1 above the more stable syn-syn. Gas electron diffraction data show that both conformers are present in gaseous O(SbMe2)2, while the presence of the syn-anti conformer in gaseous Se(SbMe2)2 is uncertain; least-squares refinements yielded the mole fraction χ = 0.27(18). The Sb-O and Sb-Se bond distances are 197.6(14) and 255.1(5) pm respectively, the valence angles of the syn-syn conformers are <SbOSb = 122.3(16)° and <SbSeSb = 96.3(11)°. It is suggested that the wide <SbOSb angle is due to across-angle repulsion between the Sb atoms.