Homoionic-montmorillonite-p-aminoazobenzene intercalation compounds were prepared by novel solid-solid reactions at room temperature. When Na-, Ca- and Ni-montmorillonites were used as host materials, the absorption maxima of p-aminoazobenzene changed considerably upon intercalation, causing the difficulty of studying photochemical behavior of the intercalated dyes. The n-dodecylammonium-montmorillonite-p-aminoazobenzene intercalation compound gave an absorption spectrum similar to that observed for p-aminoazobenzene dissolved in benzene. The intercalated paminoazobenzene in the interlayer space of n-dodecylammoniummontmorillonite exhibited reversible cis-trans photoisomerization by irradiation of UV light.