The signal of an extremely air sensitive and thermally unstable intermediate, four-membered ring oxaphosphetane in the Wittig reaction was detected for the first time at subambient temperature using liquid secondary ion mass spectrometry (LSIMS) and atmospheric pressure ionization (API) mass spectrometry. A copper probe and interface were constructed to perform LSIMS analysis at the temperature below -70 o C. A stable signal from a oxaphosphetane ion [in dry tetrahydrofolate (THF)] was obtained by LSIMS without using a viscous matrix. The signal of the oxaphosphetane was also detected using a low-temperature API source (-40 o C) connected to a reaction vessel.