To understand H-/D- negative ion formation mechanism, isotope effects of plasma production and H-/D- production in volume negative ion sources are studied. Axial distributions of H- and D- densities in the source are measured directly by laser photodetachment method. In particular, experimental results concerning dependence of H-/D- density distributions on plasma parameters are presented and discussed. Isotope effect of plasma production in H-2 and D-2 discharge is observed. H-and D- densities have different spatial distributions, respectively, corresponding to those different plasma parameter conditions. (c) 2005 Elsevier B.V. All rights reserved.