Strong mass signals of H2− and D2− ions have been observed from low-pressure dielectric barrier discharge hydrogen and deuterium plasmas via molecular beam mass spectrometry. The observed H2−/H− and D2−/D− ratios (～0.35–0.4) are over five orders of magnitude higher than those observed by other techniques. The kinetic energy of H2− and D2− ions sampled from the plasmas was determined to be widely distributed, from a few eV to >100 eV, giving lifetimes greater than ∼40 μs for H2− and ∼55 μs for D2− . The highest vibrotational excitation of neutral H2 species in the plasma was determined to be about J = 0, v = 5 or J = 19, v = 0 via threshold ionization mass spectrometry. The possible pumping mechanisms for generating H2− with further high J, required by the current high-rotation model, have been proposed. Similar to the lifetime of D2− determined recently by another group, the H2− lifetime observed in this work is about two orders of magnitude longer than that predicted by the current theoretical model. To explain these experimental observations regarding the meta-stability of long-livedH2− andD2− ions, the improved current high-rotation model or other new models, including the possible existence of some long-lived electronically excited states of H2−/D2−, need to be developed.