Radiation-induced NH3Cl and NH3Br defect centers have been studied in ammonium halide single crystals by EPR and optical spectroscopy. The defects are most likely formed by removal of H0 from an ammonium ion with subsequent bonding of NH3+ to X- along a <111> direction. In the NH3Cl center, NH3+ is a nearly planar group loosely bonded to a Cl-; in the NH3Br center there is more tendency toward sp3 hybridization of the N orbitals, and stronger covalency within the nitrogen-halogen bond. The molecular orbital description of these centers shows a strong similarity to that of mixed VK color centers in the alkali halides. Optical absorptions due to the NH3Cl and NH3Br centers were observed at 3.31 and 3.14 eV, respectively. The NH3Cl defects were observed to undergo thermally induced reorientations which are strongly influenced by the ordering of the surrounding ammonium ions at low temperature.