We have determined the molecular structure of black films of various phospholipids using X-ray reflectivity. This technique measures the electronic density profile perpendicular to the film. It allows us to accurately determine the structure of the lipidic film, i.e., to measure the density and the thickness of the zone occupied by either the aliphatic chains or the hydrophilic groups and to quantify the amount of water in the center of the film. We have studied the relations between the chemical nature of the lipids and the structural properties of the film. In particular, for zwitterionic lipids, we show that the presence of water is controlled mainly by the chemical nature of the lipid headgroup and not by the electrostatic interactions.