[en] Electron spin resonance studies have been carried out on Cu2+ ion doped in single crystals of cadmium maleate dihydrate at 303 and 77 K. It has been found that Cu2+ enters this lattice interstitially. The spin Hamiltonian parameters have been evaluated and the ground state wave function is found to be predominantly ket X2-Y2 with a slight admisture of ket 3Z2-r2. (author)