The effect of Ca 2+ ions on the dynamic properties of lipid bilayers has not been systematically studied before. Our analysis of the MD trajectories showed that the lipid molecules become less diffusive upon Ca 2+ binding, for both the zwitterionic and negatively charged lipid bilayers. However, the effect is signiicant for bilayers containing negatively charged lipids, but only minor for zwitterionic lipid bilayers. We attribute this to the electrostatic ªglueº effect of Ca 2+ on the negatively charged lipids, which makes those lipids more sticky and less diffusive. It appears that when the concentration of Ca 2+ reached a certain threshold (200 mM in our simulations), the effect became saturated, probably because the binding between Ca 2+ and lipids was saturated above the threshold and therefore the bilayer did not adsorb more Ca 2+ to become stickier.