(S)-Bromoenol lactone-d7 ((S)-BEL-d7) contains seven deuterium atoms at the 2, 3, 4, 5, 6, 7, and 8 positions. It is intended for use as an internal standard for the quantification of (S)-BEL by GC- or LC-mass spectrometry (MS). The phospholipases are an extensive family of lipid hydrolases that function in cell signaling, digestion, membrane remodeling, and as venom components. The calcium-independent phospholipase A2 (iPLA2) are a PLA2 subfamily closely associated with the release of arachidonic acid in response to physiologic stimuli. (S)-Bromoenol lactone ((S)-BEL) is an irreversible, chiral, mechanism-based inhibitor of iPLA2β that inhibits the vasopressin-induced release of arachidonate from cultured rat aortic smooth muscle (A10) cells with an IC50 value of 2 µM. (S)-BEL is more than 1,000-fold selective for iPLA2 versus cPLA2, and is 10-fold selective for iPLA2β versus iPLA2γ.