The infrared spectra of ammonia-borane, BH 3NH 3, and two of its deuterated isotropic species, BD 3ND 3 and BH 3ND 3, isolated in argon matrix at liquid hydrogen temperature have been measured. Well resolved bands for these three isotopic species have been observed for all the fundamentals. A complete frequency assignment based on C 3v molecular symmetry has been made. A set of force constants have been calculated from the data for the two isotopes BH 3NH 3 and BD 3ND 3 using a valence force field. The agreement between experiment and frequencies calculated from these force constants for the mixed isotopic species, BH 3ND 3, substantiates the present assignment.