The geometric structures of N,N-bis(trifluoromethyl)-O-(trimethysilyl)hydroxylamine, Me3SiON(CF3)(2) (1), and of trimethylsilyl nitrate, Me3SiONO2 (2), have been determined by gas electron diffraction and quantum chemical calculations (HF/3-21G*, B3LYP/6-31G*). Both compounds possess C-s symmetry. In 1 the Si-O bond is oriented syn with respect to the nitrogen lone pair (anti with respect to the CNC bisector of the N(CF3)(2) group). In 2 the Si-O bond is coplanar with the nitrate group. The following skeletal parameters (r(a) values with 3 sigma error limits) have been derived : for 1, Si-O 1.723(8) Angstrom, N-O 1.450(18) Angstrom, Si-O-N 113.4(19)degrees; for 2, Si-O 1.715(4) Angstrom, N-O 1.383(5) Angstrom, Si-O-N 120.1(9)degrees. The unusually long Si-O bonds in both compounds can be rationalized by the electron-withdrawing power of the N(CF3)(2) and NO2 groups. This leads to a reduction of the polarity of the Si+-O- bond and to lengthening of these bonds. The siloxane 2 possesses an extremely short I, l-nonbonded contact between silicon and the cis-standing oxygen of the nitrate group. Si ... O = 2.70 Angstrom. which indicates a strong intramolecular Si ... O donor interaction.