The gas-phase structure of chlorodifluoronitrosomethane, CClF2NO, has been determined by electron diffraction and calculated ab initio. Theoretically, CClF2NO is predicted to consist of two conformers having C-s (phi = 0 degrees) and C-1 (phi = 105 degrees) symmetry, which are near degenerate in energy, Delta E = 1.1 kJ mol(-1) [TZ2P/MP2 + ZPE(DZP/ MP2)], and separated by a barrier of around 1 kJ mol(-1). Equivalent C-1 conformers are predicted to be connected by a barrier of around 5-10 kJ mol(-1). The low predicted barriers to interconversion of the two conformers suggest that the rotation of the nitroso group can be regarded as being barely restricted over most values of the torsional angle at room temperature. This conclusion is supported by the gas-phase electron diffraction data, for which a dynamic model employing 11 conformations was needed to obtain an accurate fit to the experimental data. The final refined values of structural parameters for the two conformers (C-s/C-1) are (r(alpha)/pm, <(alpha)/deg) as follows : C(1)-N(2) 156.7(5)/155.9(5), N(2)-O(3) 117.5(3)/117.9(3), C(1)-Cl(4) 173.9(2)/1742(2), C(1)-F 132.0(2)/132.1(2) and 131.0(2), C(1)-N(2)-O(3) 110.8(12)/110.7(12), N(2)-C(1)-C1(4)1 17.5(5)/108.9(5), N(2)C(1)-F 103.7(2)/104.2(2) and 111.6(2), Cl(4)-C(1)-N(2)-F 123.6(14)/119.2(14) and 123.7(14).