Atmospheric pressure chemical ionization liquid chromatography/mass spectrometry was used in the identification of diacylglycerol and triacylglycerol (TAG) molecular species in a sample of a structured lipid. In the study of acylglycerol standards, the most distinctive differences between the diacylglycerol and TAG molecules were found to be the molecular ion and the relative intensity of monoacylglycerol fragment ions. All saturated TAG ranging from tricaproin to tristearin, and unsaturated TAG including triolein, trilinolein, and trilinolenin, had ammonium adduct molecular ions [M + NH 4 ] + . Protonated molecular ions were also produced for TAG containing unsaturated fatty acids and the intensity increased with increasing unsaturation. Diacylglycerol fragment ions were also formed for TAG. The ammonium adduct molecular ion was the base peak for TAG containing polyunsaturated fatty acids, whereas the diacylglycerol fragment ion was the base peak for TAG containing saturated and monounsaturated medium-and long-chain fatty acids; the relative intensities of the ammonium adduct molecular ions were between 14 and 58%. The most abundant ion for diacylglycerols, however, was the molecular ion [M − 17] + , and the relative intensity of the monoacylglycerol fragment ion was also higher than that for TAG. These distinctive differences between the diacylglycerol and TAG spectra were utilized for rapid identification of the acylglycerols in the sample of a structured lipid.Lipase-catalyzed interesterification has been used in lipid modification for the production of different kinds of lipids according to specific requirements, for instance the enrichment of triacylglycerols (TAG) with polyunsaturated fatty acids, especially essential fatty acids (1,2), and the incorporation of medium-or short-chain fatty acids into TAG (3-5). Since the interesterification alters the fatty acid composition and distribution in the acylglycerols, a group of new TAG is produced in the interesterification. Diacylglycerols are formed as the intermediates and are also found in the interesterified products; the level of diacylglycerols varied in different interesterification processes or under different reaction parameters (6,7).Reversed-phase-high-performance liquid chromatography (RP-HPLC) is a practical method for the separation of TAG molecular species according to the differences in carbon numbers and unsaturation levels of fatty acids in TAG (8-12). Equivalent carbon number (ECN) or partition number, a summary of both the carbon number and the degree of unsaturation of fatty acids, is commonly used for the tentative identification of TAG (8)(9)(10)13,14). However, it was difficult to distinguish diacylglycerols from TAG on the RP-HPLC using the ECN. Even though it is possible to separate and identify the diacylglycerol and TAG molecular species by thin-layer chromatography followed by HPLC fractionation and analysis using gas-liquid chromatography (9), the procedure is tedious and seldom used.Mass spectrometry (MS) is a sensitive method for...