Interactions of calcium ions with a monolayer of synthetic phospholipid 1,3-dihexadecylglycero-2-phosphoethanolamine (2C16PE) were investigated with three different methods : surface-pressure (pi-A) isotherms, electrochemical reactions on an electrode, and quartz-crystal microbalance (QCM) measurements. pi-A isotherms showed that the 2C16PE monolayer exists with the most condensed molecular packing at pH 5.8 probably due to intermolecular zwitterionic interactions. Ca2+ ions destroy this interaction and expand the monolayer packing. This behavior can be examined on a bilayer-covered SnO2 electrode in electrochemical methods. Oxidation peak currents of a maker ion (Fe(CN)6(3/4-)) in the aqueous phase through the lipid films were increased by the addition of Ca2+ ions only when the membrane was in a bilayer and in the fluid liquid crystalline state above the T(c). The binding behavior of Ca2+ ions onto the 2C16PE LB films was followed directly from the frequency changes of the LB film-deposited QCM. Ca2+ ions can penetrate into the fluid liquid crystalline LB films, but not into the solid films. From these findings, Ca2+ ions can bind and penetrate into only the fluid liquid crystalline state of a bilayer and disturb or expand the membrane structures.