The topotactic reaction of zinc hydroxide nitrate with aqueous solutions of CoCl 2, NiCl 2, and ZnCl 2 yields perfect pseudomorphs of (Zn, Me) 5(OH) 8Cl 2·aq after zinc hydroxide nitrate. With MnCl 2 and NaCl solutions, the nitrate is exchanged by chloride but neither Na + nor Mn 2+ is built into the lattice of zinc hydroxide chloride. The pseudomorphs give an X-ray diffraction pattern which is, disregarding a certain amount of disorder, identical with the single crystal pattern of zinc hydroxide chloride. The relative orientation of the lattices is given by: (100) Zinc hydroxide nitrate is parallel to (001) zinc hydroxide chloride and  is parallel to . The optical spectra of the pseudomorphs reveal that Co 2+ and Ni 2+ occur in tetrahedral and octahedral coordination as well. Therefore, these ions are mainly built into the intermediate layer where they occupy tetrahedral sites, but also into the close packed main layer where they are coordinated by six hydroxide ions. The atomic displacements necessary in the course of the reaction are deduced from the crystal structures of the two compounds. The mechanism of the topotactic reaction is believed to consist in oriented nucleation and crystal growth.