Carbon-13 NMR has been used to show that the deoxygenation of isocyanates to isonitriles with t-butyldiphenylsilyl lithium proceeds via the intermediates (7 and 8). The silyl carboxamide (1) can be isolated but is thermally unstable rearranging to a mixture of silyl formamides (2). The spectroscopic properties of the silyl carboxamide (1) were found to be substantially different from those previously claimed for such compounds.