Differential thermal analysis and X-ray diffraction techniques were employed to study the phase diagrams of griseofulvin-succinic acid systems. The complications of the formation of a thermally decomposed product of succinic acid, succinic anhydride, on the thermograms were discussed. The appearance of typical eutectic peaks and X-ray diffraction peaks of griseofulvin in the resolidified preparations containing 10% or less of griseofulvin indicates that the binary system is a simple eutectic mixture with negligible mutual solid solubilities. This result is partly different from that obtained previously by the microthermal microscope technique, which showed the existence of an extensive solid solution of griseofulvin in succinic acid. The formation of a solid solution was previously concluded to be primarily responsible for the enhanced dissolution of griseofulvin.