In this work, the structure of solid-supported lipid multilayers exposed to increased hydrostatic pressure was studied in situ by X-ray reflectometry at the solid liquid interface between silicon and an aqueous buffer solution. The layers' vertical structure was analyzed up to a maximum pressure of 4500 bar. The multilayers showed phase transitions from the fluid into different, gel phases. With increasing pressure, a gradual filling of the sublayers between the hydrophilic head groups with water was observed. This process was inverted when the pressure was decreased, yielding finally smaller water layers than those in the initial state. As is commonly known, water has an abrasive effect on lipid multilayers by the formation of vesicles. We show that increasing pressure can reverse this process so that a controlled switching between multi- and bilayers is possible.