On the basis of new absorption spectra of H 2, HD and D 2 taken at liquid nitrogen temperature with a 10.5-m vacuum grating spectrograph, the dissociation limits of these molecules near 850 Å were redetermined. It has been found that previous work was more strongly affected by overlapping lines and lack of resolution than was suspected at the time. Even with the increased resolution and the elimination of many overlapping lines by the use of low temperature, none of the limits of H 2 are free from overlapping lines. But the J″ = 0 limits of HD and the J″ = 0 and J″ = 2 limits of D 2 are free, and from the reliable values for the dissociation energies in the ground states, D 00(HD) = 36 406.2 cm -1 and D 00(D 2) = 36 748.9 cm -1, have been obtained, which are estimated to be accurate to better than ±0.4 cm -1. For H 2 only an upper and a lower bound can be obtained, but it is probable that the upper bound D 00(H 2) = 36 118.3 cm -1 is close to the true value. The new D 00 values are in much better agreement with the theoretical values than the old values. A brief discussion of the dissociation energies of the excited electronic states is added.