A new layered cobalt–zinc phosphite, Co(H 2 O) 4 Zn 4 (HPO 3 ) 6 ·C 2 N 2 H 10 has been synthesized in the presence of ethylenediamine as the structure-directing agent. The compound crystallizes in the monoclinic system, space group Cc (No. 9), a=18.2090(8), b=9.9264(7), c=15.4080(7)Å, β=114.098(4)°, V=2542.3(2)Å 3 , Z=4, R=0.0323, wR=0.0846. The structure consists of ZnO 4 tetrahedra, CoO 6 octahedra and HPO 3 pseudopyramids through their vertices forming bimetallic phosphite layers parallel to the ab plane. Organic cations, which reside between the inorganic layers, are mobile and can be exchanged by NH 4 + cations without the collapse of the framework.