Molybdenyl centers are studied in single crystals of NH4Cl using a set of correlated experimental techniques like EPR, optical absorption, infrared absorption, conductivity, and dielectric loss. EPR studies on NH4Cl single crystals doped with (MoOCl5)2− are made over a wide temperature range. At room temperature the resonance due to even isotopes of Mo and the hyperfine interaction due to the odd isotopes are observed. At 77 K the ligand hyperfine interaction due to 35,37Cl is also observed. The spectra are analyzed using a suitable spin Hamiltonian. An LCAO–MO method is used to interpret all the results. In the optical absorption spectra the bands corresponding to the d–d transitions and charge transfer transitions are observed. Spin–orbit coupling constant is estimated using the EPR and optical results. Molecular orbital parameters are calculated taking into account the spin–orbit effects of the ligands also. The calculated bonding coefficients are correlated with the EPR results and the nature of binding in these complexes is discussed. The spin density on the ligand orbitals is estimated using the results of the ligand hyperfine interactions. The Mo–O stretching frequencies are observed in the infrared absorption spectrum. The evidence for the existence of the bound cation vacancies comes from the conductivity and dielectric loss experiments. The impurity–vacancy binding energy is also estimated.