High resolution synchrotron radiation powder diffraction has been applied to examine structural polymorphism in high purity oleic acid (cis-9-w-octadecenoic acid) after crystallization from the melt phase. X-ray data recorded from samples re-crystallized from the melt at = 20°C were consistent with a structure having 4 molecules to a monoclinic unit cell for both the a and γ polymorphs and lattice parameters of a = 14.158 A, b = 4.252 A, c = 42.276 A, β = 96.211°, and a = 14.986 A, b = 5.267 A, c = 44.271 A, β = 92.291°, respectively. Data on the γ to a phase transformation was followed in-situ and revealed the formation of an intermediate quasi-amorphous interfacial structure prior to the phase transition with no obvious structural relationship between the phases. Crystallization following pre-heating at ≈ 50°C showed evidence for a different structure for the low temperature solidified compound thus providing supporting evidence for the role of liquid phase structure on the resultant solid-state structure.