Lipid membranes provide a sensitive platform for a variety of biological functions, and membrane lipid distributions are carefully regulated. In this project we plan to study the interaction of Ca2+- ions with model lipid monolayers that contain a mixture of zwitterionic phosphatidylcholine (PC) and anionic phosphatidylserine (PS) such that basic Ca2+-based processes as e.g. relevant in cell signalling as during apoptosis can be accessed. The conjecture of an accumulation of Ca2+-ions into the headgroup interface of a PS-containing monolayer and the resulting changes in the headgroup hydration / interfacially associated water shall be confirmed by use of neutron reflectometry.