In liquid chromatography/mass spectrometry (LC/MS) of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), attachment of an anion to the analyte molecule is the major way of producing characteristic ions under electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) conditions. The formation of RDX cluster ions in LC/MS and the origin of the clustering agents have been studied. In order to determine whether the clustering anions originate from self-decomposition of RDX in the source or from impurities in the mobile phase, isotopically labeled RDX ((13)C(3)-RDX and (15)N(6)-RDX) and isotopically labeled glycolic acid, acetic acid, ammonium formate and formaldehyde have been used in order to establish the composition and formation route of RDX adduct ions produced in ESI and APCI sources. The results showed that, in ESI, self-decomposition of RDX plays no role in adduct ion formation; rather, RDX clusters with formate, acetate, hydroxyacetate, and chloride anions present in the mobile phase as impurities at ppm levels. In APCI, part of the RDX molecules decompose yielding NO(2) (-) species which in turn cluster with a second RDX molecule producing abundant [M+NO(2)](-) cluster ions.