Phosphatidylinositol 4,5-bis-phosphate (PIP2) is an important lipid in regulation of several cellular processes, particularly membrane fusion. We use X-ray diffraction from solid-supported multilamellar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/PIP2 samples to study changes in bilayer structure and the lyotropic phase behavior induced by physiologically relevant concentrations of PIP2. Electron-density profiles reconstructed from X-ray reflectivity measurements indicate that PIP2 strongly affects structural parameters such as lipid head-group width, bilayer thickness, and lamellar repeat spacing of DOPC bilayer stacks. In addition, at lower degrees of hydration, a few molar per cent of PIP2 facilitates stalk-phase formation and also leads to formation of a hexagonal phase, which is not observed in pure DOPC. These results indicate that the role of PIP2 in membrane fusion could be, in part, due to its effect on the properties of the lipid bilayer matrix. Furthermore, coexistence of two lamellar phases with different lattice constants is observed in single-component PIP2 samples.