Confocal fluorescence correlation spectroscopy (FCS) allows for the determination of lateral diffusion coefficients and surface densities in planar phospholipid systems. The determination of the vertical (z-) position of the laser focus relative to the phospholipid surface plane is of crucial importance for the accuracy of the confocal FCS experiment. In this work we determine for the first time this vertical (z-) position of the laser focus by a so-called "Z-scan", which is based on the determination of diffusion times and particle numbers in 0.2 mum steps along the vertical (z-) axis. Experiments on supported phospholipid bilayers composed of dioleoylphosphatidylcholine (DOPC) and small amounts of Rhodamine Red-X 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (Rhodamine Red-X DHPE) adsorbed onto atomically flat mica and borosilicate glass demonstrate that results obtained by the Z-scan approach are significantly more precise than those results obtained when the fluorescence intensity maximum is used as an indicator in the determination of the vertical (z-) position of the sample. In addition to this basic contribution for the investigation of planar bilayer systems by confocal FCS, the lateral diffusion coefficients of Rhodamine Red-X DHPE in supported phospholipid bilayers composed of DOPC and cholesterol as well as in DOPC or dipalmitoylphosphatidylcholine (DPPC) monolayers adsorbed at a liquid-liquid interface were determined.