AbstractLinolenic acid of 95% purity was heated with excess alkali in ethylene glycol to produce cyclic fatty acids. Reaction variables, which are associated with the cyclization reaction and which were investigated, included solvent‐to‐fatty‐acid ratio, catalyst concentration, and reaction temperature, headspace gas (N2 or C2H4), and head‐space gas pressure.Yields of cyclic acids were improved by increasing solvent ratio (1.5舑6 wt basis), reaction temperature (225舑295C), and catalyst concentration (10舑100% excess). With nitrogen the optimum catalyst concentration was about 100% excess, but when ethylene was used, no increase was obtained beyond 50% excess catalyst. Yields of polymeric acids produced in the reaction generally decreased as cyclic acid yields increased, except in one instance.Higher yields of cyclic fatty acids were obtained with ethylene than with nitrogen under all comparable conditions, and increasing the ethylene pressure to as high as 500 psi improved the yield. Ethylene adds to the conjugated double bonds and is believed to give C20 fatty acids having a 1,4‐disubstituted monoene ring in the chain. The maximum yield of monomeric cyclic acids from 95% linolenic acid was 84.6%, the balance being polymeric and unreacted monomeric acids. Monomeric acids from this test contained 95% cyclic acids.