The oximatosilanes H₃SiONCMe₂, ClH₂SiONCMe₂ and H₂Si(ONCMe₂)₂ were prepared from the reaction of lithiated acetone oxime with bromosilane and dichlorosilane respectively. They have been characterised by NMR (¹H, ¹³C, ¹⁵N, ¹⁷O and ²⁹Si) and by gas-phase IR spectroscopy. The molecular structures of H₃SiONCMe₂ and H₂Si(ONCMe₂)₂ have been determined by low-temperature X-ray crystallography. Small Si–O–N angles have been detected in these structures, which are indicative of the formation of weak secondary interactions of the β-donor–acceptor type between silicon and nitrogen atoms. Important geometrical parameters: H₃SiONCMe₂, Si–O 1.673(1), N–O 1.433(1) Å, Si–O–N 106.0(1)°, NC 1.273(2) Å; H₂Si(ONCMe₂)₂, Si–O 1.657(1), 1.650(1), N–O 1.466(2), 1.447(2), NC 1.271(2), 1.266(2) Å, Si–O–N 102.5(1), 107.5(1)°. The geometries of H₃SiONCMe₂, FH₂SiONCMe₂ and ClH₂SiONCMe₂ were calculated by optimisations at the MP2/6-311G** level of theory. The Si–O–N angles in FH₂SiONCMe₂ and ClH₂SiONCMe₂ are predicted to be smaller than that in H₃SiONCMe₂. A natural bond orbital analysis describes the β-donor interaction as weak negative hyperconjugation lp(O)→σ*(SiO) and lp(O)→σ*(SiH). In general the β-donor bonds in oximatosilanes are found to be weaker than in the related hydroxyaminosilanes.