AbstractMethyl‐branched derivatives of methyl 8,11,14‐eicosatrienoate form stable liquid‐expanded monolayers. Surface areas are expanded by the methyl branch. The expansion effect is a function of surface pressure. At high surface pressure, the greatest expansion occurs with a mid‐point methyl branch. At low surface pressure, surface area increases continuously as the methyl group is moved along the carbon chain from carbon 19 to carbon 5. Desaturase activity varies inversely with surface area, and a linear correlation exists between surface area at low surface pressure and the desaturation rate. These data support the concept that lipid structure and its effect on short range forces between molecules is an important factor in desaturase activity.