Phase diagram studies have shown that 2,9-dinitrophenol forms a 1:1 addition compound with naphthalene. There isa formation of two eutectics, one between naphthalene and a congruent melting compound and the other between the congruent melting compound and 2,4-dinitrophenol. The linear velocity of crystallization and the flow time measurements of the molten material have shown some sort of molecular association between the components in the eutectics, and also there is a weak molecular interaction between 2,4-dinitrophenol and naphthalene in the addition compound. The interaction may be a dipole-induced dipole interaction. Microstructures of the addition compound are changed considerably in the presence of impurities.