Among the mixed crystals between carbazole (1) or benzimidazole (2) and an aromatic hydrocarbon, such as naphthalene, phenanthrene and anthracene, only the carbazole/anthracene mixed crystal was found by powder X-ray diffraction spectroscopy end differential scanning calorimetry to constitute a molecular compound. Irradiation of this molecular compound gave 9-(9-carbazolyl)-9,10-dihydroanthracene (7) and 9-(9-carbazolyl)anthracene (8) both in the solid state and in an acetonitrile solution. The structure of 7 was established by Xray crystallographic analysis. Benzimidazole showed no photoreactivity to aromatic hydrocarbons in the solid and solution states, except the irradiation with 2,3-dimethylnaphthalene in acetonitrile leading to the formation of 1-(1-benzimidazolyl)-2,3-dimethylnaphthalene (9).