Cu2+ and VO2+ doped ammonium hydrogen oxalate hemihydrate, [(NH4)HC2O4 center dot(1)/2H2O], single crystals have been studied at room temperature and at 113 K in three mutually perpendicular planes. Both ions yield unexpectedly large number of lines. The calculated results of the Cu2+ and VO2+ doped in [(NH4)HC2O4 center dot(1)/2H2O] indicate that both ions substitute with the NH4+ ion in the structure. The EPR spectra of Cu2+ ions are characteristic of tetragonally elongated octahedral site and the spectra of VO2+ are characteristic of tetragonally compressed complex. The angular variation of the EPR spectra has shown that two different Cu2+ and VO2+ complexes are located in different chemical environments, and each environment contains two magnetically inequivalent Cu2+ and VO2+ sites in distinct orientations occupying substitutional positions in the lattice and show very high angular dependence. The principal g and the hyperfine (A) values of both ions are determined. (c) 2005 Elsevier B.V. All rights reserved.