The geometric structure and conformational properties of bis(fluorooxy)difluoromethane, CF2(OF)(2), were determined by gas electron diffraction and theoretical methods (ab initio and density functional calculations). The electron diffraction intensities are reproduced best by a mixture of 70(10)% (+sc,+sc) and 30(10)% (sc,ap) conformers, corresponding to Delta H degrees = H degrees(sc,ap) - H degrees(+sc,+sc) = 0.9(3) kcal mol(-1). This experimental result is reproduced very well by HF/6-31G* and MP2/6-31G* calculations (Delta E = 0.7 kcal mol(-1)), but not by the HF/3-21G and the density functional (SVWN/6-311G*, BLYP/6-311G*, B3LYP/6-311+G*) approximations. In addition, the conformational properties of CF2(OH)(2) and CH2(OF)(2) were investigated theoretically. The experimental and theoretical results are discussed in terms of anomeric effects.