The molecular packing in different polymorphic forms of a stereospecific triacylglycerol, 1,2-dipalmitoyl-3-decanoyl-sn-glycerol (PP10), was examined by X-ray diffraction, differential scanning calorimetry, and infrared and Raman spectroscopy techniques. On quenching, the isotropic liquid produced a hexagonally packed α-phase, which upon heating melts at 22°C and transforms into a bilayered (37 Å) β′3-phase in which the chains pack in an O⊥ subcell. At 34°C the β′3 form transforms into another bilayered β′2-structure with similar long spacing periodicity, but different chain packing, O⊥ subcell. The β′2-phase melts at 39°C to an isotropic liquid. However, on incubation of this phase at 37°C for 45 h, a trilayered (56 Å) β-phase with chains packed in a triclinic parallel (T∥) subcell is nucleated, which grows on prolonged incubation. An additional β′-phase (β′1) is produced from the α-phase melt. On rapid heating to 39°C the α-phase melts and on incubation produces a hexalayered (111 Å) β′1-phase with O⊥ subcell, which melts to an isotropic liquid at 42.5°C. The α ⇒ β′3 and β′3 ⇒ β′2 transformations are bilayer to bilayer transitions, where the decanoyl and palmitoyl chains are packed together in each layer but with different lateral chain packing. During the β′2 ⇒ β transition the chain packing changed from perpendicular to parallel and was accompanied by chain segregation to form a trilayer structure. In the β-phase the decanoyl chains segregate in a layer between two layers of palmitoyl chains. The thermodynamic parameters of the T∥ packing compared to that of equal carbon numbered trimyristin suggest that this phase is only marginally more stable than the O⊥ packing. The vibrational spectra confirm the lateral chain packing present in these polymorphic forms. The Raman spectral data shows that the β-phase has an unusually high gauche/trans ratio compared to that of the β′1-phase. The infrared spectra indicate perturbation in the carbonyl region and in the end methyl plane region of all the three β′-phases that pack in an O⊥ subcell.