The multistage character of the reaction of dimethyl(chloromethyl)chlorosilane (I) with N-trimethylsilyl-amides and -lactams (II) was shown by NMR monitoring. Interaction of the reactants starts with transsilylation, leading to Me3SiCl and the corresponding N-[dimethyl(chloromethyl)silyl]amide or -lactam (III). The second stage, intramolecular (dimethylchlorosilyl)methylation of unstable III, proceeds in two directions. A kinetically controlled transformation of III affords previously unknown O-(dimethylchlorosilyl)methylated intermediates, O-[(dimethylchlorosilyl) methyl]imidates (IV) containing a hypervalent ClSiN bond. The already known products, (OSi) chelate N-[(dimethylchlorosilyl)methyl]amides (V), arise from the reaction carried out under thermodynamic control. The same compounds are also formed via the Chapman rearrangement of the intermediates (IV).