The lamellar-to-hexagonal phase transition of a phospholipid/ surfactant mixed system of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and oligo(ethylene oxide)dodecyl ethers of type C12H25O(CH2CH2O)(2)H (C12E2) in molar surfactant/phospholipid ratio (R-S/L) Of 2 at low hydration driven by temperature has been studied by X-ray diffraction, The L-beta-H-II phase transition is a reversible two-state process showing hysteresis at fast temperature scan rates, The obtained hexagonal phase exhibits a temperature dependent structural change. The numbers of bound water molecules per composite particle (WS+L,) absorbed in the lamellar and hexagonal phases are nearly the same, changing from WS+L = 5.0 to 4.7 during the phase transition. The fluidity of the alkyl chains on increasing the temperature and the close packing of the hydrophilic molecular parts are the driving parameters of the lamellar-to-hexagonal transformation,