Vitamin D plays important roles in the bone formation, in calcium and phosphorus homeostasis and in the treatment and prevention of many diseases. Ions, especially divalent cations like Mg2+, have indispensable roles in many vital biological events. Mg2+ is involved in many fundamental processes such as stabilization of membranes and macromolecules, synthesis of nucleic acid and proteins and formation and use of high-energy phosphate bonds. Mg2+ is also required for synthesis of more than 310 different enzymes of the body and is, therefore, involved in many important activities. The roles of vitamin D and major ions in the body are quite well known. While there are still many unresolved points about the exact molecular mechanism behind such diverse functions, in the present study, the interaction of Mg2+ with dipalmitoyl phosphatidylcholine (DPPC) model membranes has been studied in the presence and absence of vitamin D, by using Fourier transform infrared (FTIR) spectroscopy and turbidity technique at 440 nm. The effect of different buffer media on the system has also been investigated. The temperature dependent investigation of the wavelength of the CH, antisymmetric stretching bands revealed that, in the presence of N-[2-hydroxyethyl] piyerazine-N'-[2-ethanesulfonic acid] (Hepes) and phosphate buffer, addition of Mg2+ and/or vitamin D, into pure DPPC liposomes does not change the shape of the phase transition profile. Turbidity studies support these results. In the presence of Hepes buffer, the inclusion of Mg2+ and/or vitamin D, into pure DPPC liposomes orders the system. In the presence of phosphate buffer, FTIR study showed that, addition of Mg2+ into pure DPPC liposomes disorders the system in the gel phase. The precipitation of Mg2+ with phosphates, which is present in phosphate buffer, may be a reason for this difference in the effect. It is seen that, the binary mixture of Mg2+-DPPC and the ternary mixture of Mg2+-vitamin D-2-DPPC behave differently in the presence of two different buffer media. (C) 2000 Elsevier Science B.V. All rights reserved.