An apparatus for measuring the deuterium and tritium in erbium hydride films on neutron tube targets has been designed, constructed, and tested at the Tritium Science and Fabrication Facility at Los Alamos National Laboratory. The apparatus consists of a pumped thermal desorption inlet system (TDIS) and a high-resolution mass spectrometer (HRMS). The pumped TDIS has a unique capability to perform complete and rapid thermal desorption of gases from the erbium hydride film and pressure-volume-temperature measurement for accurate quantification of the total moles of released gas. The HRMS measures the partial pressures of the hydrogen isotopes in the released gas. From these two measurements, we can determine the total moles of each hydrogen isotope in erbium hydride films accurately. The optimum desorption time is 2 to 3 minutes; the optimum desorption temperature is 900 degreesC. Under these conditions, 99.98% of the total moles of deuterium in the hydride films of deuterated erbium (ErD2) targets are released in a single desorption step. The molar atomic ratios of total hydrogen isotopes to erbium (HDT/Er), deuterium to erbium (D/Er), and tritium to erbium (T/Er) of deuterated and tritiated erbium (ErDxT2-x) targets are measured with better than 2% precision. (C) 2004 American Vacuum Society.