Two negative ion sources for which H2(v‘) vibrational spectra have been reported are studied with a numerical model. The model gives reasonably good agreement with available experimental measurements, with the notable exception of the vibrational spectra. There are significant quantitative differences at the higherv‘levels, but also some intriguing qualitative similarities.A scaling study of high power sources is also reported. Gas dissociation and possibly vibrational cooling by thermal electrons appear to explain the saturation and in negative ion production with increasing input power. The surface‐to‐volume ratio becomes an important parameter for high power sources, since it affects the atomic recombination rate and the plasma density.