A qualitative analysis for the ammoximation of acetaldehyde to its oxime in the TS-1(Titanium Silicalite-1)/H2O2 system was investigated using an in situ infrared spectrometer (ReactIR15). NH3 is first oxidized to NH2OH by TS-1/H2O2; then, CH3CH=NOH forms after NH2OH reacts with CH3CHO. That means the intermediate of this reaction is NH2OH instead of CH3CH=NH. Experiments have been conducted to verify the mechanism, and the results are in good agreement with the infrared findings.