The thermal changes induced in dipalmitoyl phosphatidyl glycerol (DPPG) model membranes were studied by FIR technique. When temperature increases, the DPPG model membrane passes from a high ordered gel phase to a few ordered liquid crystalline phase. The very quick passing between the two phases is characterized by the main phase transition temperature at which the bilayers contain equal percentages of ordered and disordered phospholipids. The main phase transition induces important changes in the symmetric and asymmetric stretching vibrations of the -CH2 groups of the DPPG acyl tails. Mathematical methods for determining the main phase transition and the degree of order of the DPPG model membrane at a given temperature are proposed in this paper. (c) 2008 Elsevier B.V. All rights reserved.