The nitrogen and hydrogen vibrational modes of hydrogenated Ga As 1− y N y and Ga P 1− y N y have been studied by infrared absorption spectroscopy and density functional theory. Data for the stretching modes observed for samples containing both hydrogen and deuterium show that the dominant defect complex contains two weakly coupled NH stretching modes. Theory predicts an HNH complex with C 1 h symmetry whose vibrational properties are in excellent agreement with experiment. Additional results provide further support for the defect model that has been proposed. Uniaxial stress results confirm that the symmetry of the HNH complex must be lower than trigonal. The vibrational properties predicted by theory for the HNH complex also lead to an assignment of the wagging modes that are observed. Experimental and theoretical results for Ga As 1− y N y and Ga P 1− y N y are remarkably similar …