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The gas--solid reaction of polycrystalline 1,1 diphenylurea with hydrogen bromide and deuterium bromide was investigated in the infrared region between $4000-400cm-1$. A decrease in the intensity of infrared bands associated with N-H vibrations was observed along with an increase in the intensity of a band at $3153cm-1$ assigned to the $NH3+$ stretching vibration. Evidence is presented for the addition of hydrogen bromide to the $-NH2$ group of 1,1 diphenylurea.