The paramagnetic centres of Cu 2ฯฉ in ammonium bromide crystals were studied at 300 and 77 K with X-band and Q-band EPR. We determined the spin Hamiltonian parameters, contact interaction and covalent bonding parameters of the paramagnetic centres of the NH 4 Br:Cu 2ฯฉ . The superhyperfine interaction to the Cu paramagnetic centres in NH 4 Br is much stronger than in NH 4 Cl. The results obtained for the contact interaction (with a new assignment that we gave to the EPR spectra) are in agreement with those found for other copper complexes. The comparison of the results of the covalent bonding parameters with those of other compounds showed a strong covalency of NH 4 Br:Cu 2ฯฉ , suggesting that orbital admixtures in this complex are large and apparently much bigger than estimates based on overlap effects alone with no covalent bonding. The Cu 2ฯฉ wave function of the NH 4 Br written as an admixture of อ3z 2 ฯช r 2 อ and อx 2 ฯช y 2 อ orbitals is not suitable to describe the system. A real model must take into account the admixtures with the ligand orbitals. The d-s mixing is high in the CuBr 4 (NH 3 ) 2 2ฯช centre, indicating strong bonding between the Cu 2ฯฉ ion and the two NH 3 molecules. In the case of the CuBr 4 (NH 3 )(H 2 O) 2ฯช and CuBr 4 (H 2 O) 2 2ฯช centres the binding between the Cu 2ฯฉ ion and the H 2 O molecules is much weaker.