10-oxo-12(Z)-Octadecenoic acid-d5 is intended for use as an internal standard for the quantification of 10-oxo-12(Z)-octadecenoic acid by GC- or LC-MS. 10-oxo-12(Z)-Octadecenoic acid is a metabolite of linoleic acid and an activator of transient receptor potential vanilloid 1 (TRPV1).^1,2 It is formed from linoleic acid by conjugated linoleic acid dehydrogenase (CLA-DH) via a 10-hydroxy-12(Z)-octadecenoic acid intermediate and can also be produced from linoleic acid by gut microbiota.¹ 10-oxo-12(Z)-Octadecenoic acid (100 µM) selectively increases calcium levels in HEK293 cells expressing TRPV1 over those expressing TRPV2, TRPV3, TRPV4, and TRP melastatin 8 (TRPM8).² It also induces inward currents in HEK293 cells expressing TRPV1, an effect that can be blocked by the TRPV1 antagonist capsazepine. Dietary administration of 10-oxo-12(Z)-octadecenoic acid (0.1% w/w) reduces weight gain and adipose tissue weight and increases the expression of the gene encoding mitochondrial uncoupling protein 1 (Ucp1) in wild-type, but not Trpv1 knockout, mice fed a high-fat diet. It also decreases plasma glucose and triglyceride levels in diabetic KKAy mice fed a high-fat diet.