We present the results of a numerical study of isotope effects in volume negative ion sources. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a multicusp source. A predicted peak in negative ion production Te‐opt≊0.6–0.8 eV finds support from our analysis of D−and H−densities measured in a hybrid multicusp source. The most significant isotope effect suggested by this modeling effort is a stronger V–T cooling of the vibrational distribution in deuterium, which reduces negative ion production in high power deuterium discharges below that in equivalent hydrogen discharges.