Abstract By DTA, X-ray diffraction and IR spectrometry, it has been shown that triesters (RCOOCH 2) 3CC 2H 5 (RCOOC 12, C 14, C 16 and C 18) are monotropic, and can exist under four crystalline species: α, β′ I, β′ II and β, instead of α, β′ and β for the corresponding triglycerides. β′ I and β′ II have the same melting temperatures. β′ II differs from β′ I only by a small difference in the angle of tilt of hydrocarbon chains on their basic plane, and probably by a regular arrangement of ethyl groups. The arrangement of pitchfork-shaped molecules is head-to-tail in these structures, and long spacings correspond to two (α, β′ I and β′ II) of three (β and exceptionally β′ I for the dodecanoic derivative) chain lengths. Two νCO bands are encountered in the IR spectra of all forms. The C 16 derivative presents a transient sub β′ form. A supplementary study of the trioctadecanoate of tris(hydroxymethyl)-1,1,1 pentane has shown that the alkyl group substituted on the central carbon atom of the structure is responsible for the existence of the two β′ forms (of which melting temperatures are obviously distinct in this case). Moreover, this group seems to have a tendency to become parallel with other hydrocarbon chains when its length increases.