Researches on phospholipid bilayers have importance on various scientific fields such as biophysics, colloidal chemistry and soft matter physics. Investigations on physico-chemical properties of the phospholipid bilayers, e.g., bilayer structure, phase behavior and molecular dynamics, are always essential for these researches. Here, I introduce remaining issues concerning the physico-chemical properties, and our recent approaches to them. In section 1, I simply introduce the phospholipid bilayers and our researches toward the systematic understanding of effects of additives on the bilayers, which has been published in this journal recently. Correlations between hydration states and the phase transitions of the lipids, and the interaction between ions and neutrally charged bilayers are described in section 2 and 3, respectively. Novel working mechanism of the electrostatic double-layer force found from the equilibrium lamellar repeat distance of the bilayers is described in section 4.