The work reported here demonstrates for the first time a forward and reverse topochemical, single-crystal-to-single-crystal reaction. By full crystal structure analysis of the product and substitutional crystals of intermediate conversion, it is proven that the forward topochemical photodimerization proceeds homogeneously through a solid solution of monomer and dimer over the entire range of conversion. The photoreaction is shown to be driven homogeneously or heterogeneously through choice of irradiation conditions.