Two series of compounds, HxSi(ONMe2)(4-x) (3a, 2a, 1a) and HxSi(ONEt2)(4-x) (3b, 2b, 1b) (with x = 1, 2, 3), have been prepared by either the condensation of N,N-dialkylhydroxylamines with halogenosilanes in the presence of; 2,6-lutidine as an auxiliary base or by the more selective reaction of the O-lithiohydroxylamines with halogenosilanes at low temperatures. The compounds are very sensitive to hydrolysis, but are not pyrophoric, and are Stable at ambient temperature to a potentially very exothermic rearrangement into aminosilanoles. The compounds have been characterized by gas-phase IR and solution NMR spectroscopy (H-1,C-13, N-15, O-17, Si-29) and by mass spectrometry. The IR frequencies of the simplest compound H3SiONMe2 (1a) have been assigned by comparison with ab initio frequencies. The NMR data are discussed in the light of P-donor interactions. The Si-29 NMR shifts of the series HxSi(ONMe2)(4-x) are compared with those of the isoelectronic isopropoxysilanes, HxSi(OCHMe2)(4-x), which have been prepared for this purpose. Single crystals of H3SiONMe2 (1a), H3SiONEt2 (1b), H2Si(ONMe2)(2) (2a), and HSi(ONMe2)3 (3a) have been grown by in situ methods, and their structures have been determined by X-ray diffraction. All compounds have small Si-O-N angles, with the minimum of 95.2 degrees occurring with H2Si(ONMe2)(2) (2a). The crystallographic data are compared with the results of ab initio calculations (MP2/6-311G**) and the crystal structure of the isoelectronic H3SiOCHMe2,which has now been deterinined, and the earlier reported Si(OCHMe2)(4). H3SiOCHMe2 crsstalizes in a transition state geometry according to MP2/6-311G** calculations and has a Si-O-C angle of 118.4(1)degrees. The data show clearly the large differences between Si-O-N and Si-O-C angles, which are attributed to the occurrence of P-donor interactions in SiO-N units. The strength of this interaction has been estimatedto be 15 kJ mol(-1) from ab initio calculations.