Thermotropic phase transition and stability of phosphatidylinositol 4,5-bis(phosphate) (PIP2) aggregates solubilized in buffer solution were studied by using synchrotron radiation X-ray small-angle scattering, and the modeling analysis using a double-shell-ellipsoid fitting to the experimental scattering data was carried out. At pH 6.7 the PIP2 molecules aggregated to form ellipsoidal micelles with an identical size. The modeling analysis shows that the hydrophilic region composed of the PIP2 polar heads is highly hydrated. The elevation of temperature from 10 to 50 degrees C induced a moderate shrinkage of the micellar dimension and a systematic change of the intramicellar scattering density distribution, which is considered to occur mostly in the hydrophilic portion of the micelles. In contrast to the above case, at pH 5.4 the PIP2 solution showed a disordered lamellar phase at 10 degrees C and changed drastically to a micellar phase by elevating temperature. The present result suggests a high sensitivity of the interaction between water and PIP2 polar heads to the variation pH and temperature.