The hydrazidosilanes Ph2Si(NHNMe2)2 (1), Me2Si(NHNMe2)2 (2), PhSi(NHNMe2)3 (3), MeSi(NHNMe2)3 (4), PhClSi(NHNMe2)2 (5), MeClSi(NHNMe2)2 (6), Me2ClSi(NHNMe2) (7), MeClHSi(NHNMe2) (8), MeHSi(NHNMe2)2 (9), Me2HSi(NHNMe2) (10), Me2NN[HSi(NHNMe2)2]2 (11) and Si(NHNMe2)4 (12) have been prepared by the reaction of the corresponding chlorosilanes with N,N-dimethylhydrazine. Some of the compounds containing Si–Cl and N–H functions simultaneously (6, 7, 8) are very reactive and tend to polymerise and could only be characterised by spectroscopic methods (1H, 13C, 29Si NMR, IR, MS). All other compounds could additionally be characterised by elemental analyses. The structures of 1, 2, 5, 9, 11 and 12 in the solid state were determined by X-ray diffraction. These include the first structural determinations of compounds containing SiH–NH–N (9, 11) and SiCl–NH–N (5) units.