At 123 K, the crystal structure of 1,2-dipalmitoyl-sn-glycerol (C35H6sOs, M r = 568.9) is monoclinic with space group P2 l, a = 5.480(1), b = 7.301(1), c = 43.145(7)/k, fl = 92.91(1) ยฐ, V= 1724 ,~ 3 Dc = 1.0960 g cm -3 and Z = 2. Integrated X-ray intensities for 3574 independent reflections were measured with Ni-filtered Cu-Ku radiation (h = 1.5418 ,~, /z =0.56 mm-J), and these were all used in a full-matrix least-squares refinement which gave R(F 2) = 0.142. Bond lengths and angles have e.s.d.s less than 0.01 ,~ and 0.1 ยฐ for non-hydrogen atoms. Comparison with a previously reported study of the room temperature structure shows a very similar orthorhombic mode of chain packing but significant conformational differences in the polar headgroup. These changes may occur in conjunction with small changes in enthalpy (less than 1 kJ/mol) observed at 202, 193 and 136 K and with a discontinuity in the unit cell volume observed in the range 200-190 K. Analysis of the atomic mean square displacements at 123 K indicates the presence of internal torsional vibrations in the headgroup region that involve the carbonyl and hydroxyl O-atoms.