The phase diagram of (naphthalene + n-pentacosane) has been determined by thermometrical and differential thermal analyses and X-ray diffraction carried out on the two pure compounds and twelve binary mixtures. The solids phases of the pure compounds are immiscible, but there is cosolubility in the liquid state. With increasing temperature, the binary mixtures undergo structural changes corresponding to those of pure n-pentacosane, while naphthalene seems to retain its initial crystalline structure until fusion occurs. The binary diagram, determined by experiments, displays an eutectic solidification with immiscibility in the solid state: liquid <----> delta(0)-C10H8 +beta(o)'(Pbnm)-C25H52.