Conformational disorder in liquid alkenes and in the L α and H ∥ phases of some unsaturated phospholipids has been monitored by FTIR spectroscopy. The CH 2 wagging region (1330–1390 cm −1) in saturated chains contains vibrations of particular 2- and 3-bond conformational states as follows: 1341 cm −1, end- gauche (eg); 1352 cm −1, double gauche (gg); 1368 cm −1, the sum of kink and gtg states. In unsaturated chains, this spectral region revealed an additional band at 1362 cm −1 and (occasionally) a feature near 1348 cm −1. The 1362 cm −1 band is tentatively assigned to the wagging of CH 2 groups adjacent to the C C bond. Substantial populations of both gg and (kink+ gtg) states are evident in the L α phases of unsaturated phosphatidylcholines (PC's). Unsaturated phosphatidylethanolamines (PE's) are more ordered than their PC counterparts, and possess fewer gg and eg states. Chain disorder in the H ∥ phase of PE's approaches that in L α phases of unsaturated PC's. Changes in conformer distributions during the L α →H ∥ transition in 1,2-dioleoylphosphatidylethanolamine (DOPE), 1-palmitoyl,2-oleoylphosphatidylethanolamine (POPE), 1,2-dielaidoylphosphatidylethanolamine (DEPE), and N-methyl-DOPE(N-MeDOPE) were semi-quantitatively estimated. For DOPE and DEPE, slight cooperative increases in both gg and (kink+ gtg) states occur, for POPE only the gg population increases and for N-MeDOPE only the kink+ gtg populations increase. These disorder increases are consistent with the small calorimetric Δ H for this transition. Difficulties in quantitative determination of conformational disorder in unsaturated chains are discussed.