The geometric structure of perfluoro-N,N-dimethylformamide, (CF3)(2)NC(O)F, has been determined by gas electron diffraction (GED) and quantum chemical calculations (HF, MP2, and B3LYP with 6-311G* basis sets). The bonds around nitrogen possess a nearly planar configuration with the sum of the nitrogen bond angles Sigma alpha(N) = 359.5(3)degrees. The (CF3)(2)N group is rotated around the N-C(sp(2)) bond by 33(4)degrees, away from the configuration in which the nitrogen lone pair is perpendicular to the C(O)F plane. The calculated (B3LYP) barriers to internal rotation around the N-C(sp(2)) bond are 7.92 kcal mol(-1) for perpendicular orientation of the (CF3)(2)N group (nitrogen lone pair parallel to the C(O)F plane) and 0.32 kcal mol(-1) for planar orientation (nitrogen lone pair perpendicular to the C(O)F plane). The barrier for perpendicular orientation is much lower than the experimental gas-phase value for the parent compound Me2NC(O)H (Delta H-not equal = 19.7(3) kcal mol(-1)). This theoretical result is in agreement with F-19 NMR spectra which show one doublet for the two CF3 groups in the temperature range 0 to -70 degrees C. Thus, fluorination causes a strong decrease of conjugation between the nitrogen electron lone pair and the pi(C=O) bond. The following skeletal parameters (r(a) distances with 3 sigma uncertainties) were derived in the GED experiment: C=O 1.182(6) Angstrom, N-C(sp(2)) 1.394(15) Angstrom, N-C(sp(3)) 1.451(4) Angstrom and N-C=O 129.4(15)degrees.