Aldehydes react with MSTFA (N-methyl-N-trimethylsilyl-trifluoroacetamide to give E/Z- silylenolethers and surprisingly addition products of the reagent to the carbonyl group. The mass spectra of the MSTFA/aliphatic aldehyde adducts are dominated by a key fragment of m/z 228 formed by elimination of the alkyl pan. With MSTFA/aromatic aldehyde adducts this fragmentation is replaced by MH and M-CH3NCOCF3 (M-126 amu) ions.