Abstract: The electronic structures of cuprous halide monomers and trimers have been calculated by CNDO/2 method.It was found that both CuXand Cu₃X₃(X=Cl,Br,I) are essentially covalent molecules.The structural formulas for the monomers and trimers may be represented as follows.The covalences of Cu in Cu₃X₃ and CuXare 5 and 3 respectively,while the covalence of Cl,Br or Iis 1.Although the electronegativies of the halogen atoms are greater than that of Cu atom,the net charges on Cu of both Cu₃X₃ and CuXare negative and that on the halogen atoms are positive,because of the presence of the dative bond X→Cu,which transfers electrons from the halogen to the copper atom.