We present an investigation of the quasibinary systems CoIn 3Àx Zn x and CoGa 3Àx Zn x which were structurally characterized by X-ray diffraction experiments and, in the case of CoGa 3Àx Zn x , additionally by neutron powder diffraction experiments. The limiting compositions were found to be x=0.81(2) and x=0.73(2) for CoIn 3Àx Zn x and CoGa 3Àx Zn x , respectively. The isotypic binary compounds CoIn 3 and CoGa 3 crystallize with the FeGa 3 structure type (tetragonal, space group P4 2 /mnm, Z=4) in which the p-block atoms form an array of columns of centered cubes defined by two different crystallographic sites. The substitution of In or Ga by Zn takes place in an ordered fashion and produces ''colored'' variants of the FeGa 3 parent structure: In both systems Zn enters exclusively the position corresponding to the cube centers. Additionally, in CoIn 3Àx Zn x this position is substituted in such a way that for a composition CoIn 2.5 Zn 0.5 , columns of Zn-and Infilled In 8 cubes along the c axis alternate. The latter substitution pattern is accompanied by a symmetry lowering of the parent FeGa 3 structure: The structure of CoIn 3Àx Zn x is described by the space group P4 2 /m in which the cube center position is split into two separate sites. By performing first-principles electronic structure calculations we investigated the general bonding situation of the compounds CoIn 3 and CoGa 3 and the particular electronic effect when incorporating Zn. With respect to the density of states of the binary compounds the exchange of Ga or In by Zn virtually affects only the electronic states just below the Fermi level. On increasing Zn concentration a dip is created in the density of states which approximately coincides with the location of the Fermi level for an electron count corresponding to limiting composition of the two systems.