Abstract Electron paramagnetic resonance studies of molybdenyl doped NH 4BR crystals were made at room temperature. The resonance due to even isotopes of Mo and the hyperfine interaction due to 95,97Mo is observed. The angular variation of the spectrum is studied in the crystallographic (001)plane. The symmetry axes of the magnetic complexes are found to be along <100>, <010> and <001> directions of the crystal. The spectra are analysed using a spin hamiltonian corresponding to tetragonal symmetry. The spin hamiltonial parameters indicate the formation of a stable[MoOCl 5] 2− complex in the lattice. The ligand spin orbit effects on g and A values is discussed. The spin hamiltonian parameters are correlated with the ligand properties like electronegativity, covalent nature, etc.