AbstractMethylcis,cis‐,trans,cis‐,cis,trans‐, andtrans,trans‐12,15‐octadecadienoates‐9,10‐d2were prepared by the Wittig Reaction betweencis‐ ortrans‐3‐hexenyltriphenylphosphonium bromide and methyl 12‐oxododecanoate‐9,10‐d2with butyl lithium in ethyl ether. At room temperature, the Wittig Reaction gave a product mixture with 80‐85%cisgeometry in the double bond formed. When the reaction was run at −40°C, followed by protonation with methanol and slow warm‐up to room temperature, the product mixture had 60‐65%transgeometry in the double bond generated. Mixtures of thecis,cisandtrans,cisand of thecis,transandtrans,transisomers were separated by partial silver resin chromatography, and the individual pure fatty esters were converted to the triglycerides through the respective acid chlorides. Melting points and nuclear magnetic reson