A quantitative liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) method was developed for the analysis of the explosive, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). In negative ionization mode, HMX forms an acetate adduct ion [M + CH3COO] -, m/z 355, in the presence of a small amount of acetic acid in the mobile phase. The ESI collision-induced dissociation (CID) spectrum of m/z 355 was acquired and the transitions m/z 355 → 147 and m/z 355 → 174 were chosen for the determination of HMX in samples. Using this quantification technique, the method detection limit was 1.57 μg/L and good linearity was achieved in the range 5-500 μg/L. This method will help to unambiguously analyze environmentally relevant concentrations of HMX.