High resolution infrared emission spectra of beryllium monohydride and monodeuteride have been recorded. The molecules were generated in a furnace-discharge source, at 1500 degreesC and 333 mA discharge current, with beryllium metal and a mixture of helium and hydrogen or deuterium gases. Approximately 160 BeH lines and 167 BeD lines for the vibrational bands v=1-->0 to v=4-->3 were observed in the spectra and spectroscopic constants were determined. The Dunham constants (Y-l,Y-m) and Born-Oppenheimer breakdown constants were obtained in a combined fit of the BeH and BeD data. The equilibrium rotational constants (B-e) for BeH and BeD were found to be 10.319 59(3) cm(-1) and 5.688 29(2) cm(-1), respectively, while the equilibrium vibrational constants (omega(e)) are 2061.416(3) and 1529.956(3) cm(-1). The equilibrium distance (R-e) was determined to be 1.342 436(2) Angstrom for BeH. (C) 2003 American Institute of Physics.