The infrared spectra of sublimed films of PH4I and PD4I were obtained at —78° and —190°C in the region 4000–400 cm—1. Powder and Nujol mull spectra were obtained at room temperature. The low temperature spectra could be interpreted on the basis of a Vd site symmetry for the PH4+ ion as occurs in the phase III NH4Br and NH4I structures. The room temperature spectra are also compatible with the room temperature x-ray structure, D4h7. No evidence for any sort of transition was found. The hydrogen bond energy in PH4I is less than 0.5 kcal/mole.