The monolayer properties of some single-chain polyprenyl phosphates (phytanyl, phytyl, and geranylgeranyl phosphates), which we regard as hypothetical primitive membrane lipids, were investigated at the air-water interface by surface pressure-area (π-A) isotherm measurements. The molecular area/pressure at various pH conditions dependence revealed the acid dissociation constants (pK(a) values) of the phosphate. The pKa values thus obtained at the air-water interface (pK(a1) = 7.1 and pK(a2) = 9.4 for phytanyl phosphate) were significantly shifted to higher pH than those observed in the bilayer state in water (pK(a1) = 2.9 and pK(a2) = 7.8). The difference in pK(a) values leads to a stability of the phosphate as both monolayer and bilayer states in a pH range of 2-6. In addition, the presence of ions such as sodium, magnesium, calcium, and lanthanum in the subphase significantly altered the stability of the polyprenyl phosphate monolayers, as shown by the determination of monolayer collapse and compression/expansion hysteresis. Although sodium ions in the subphase showed only a weak effect on the stabilization of the monolayer, addition of magnesium ions or of a small amount of calcium ions significantly suppressed the dissolution of the monolayer into the subphase and increased its mechanical stability against collapse. In contrast, the presence of larger amounts of calcium or of lanthanum ions induced collapse of the monolayers. Based on these experimental facts, a plausible scenario for the formation of primitive cell membrane by transformation of a monolayer to vesicle structures is proposed.