Measurements of plasma parameters, including electron density, electron energy distribution function (eedf), and negative ion density, have been made in the driver and extractor regions of a multicusp tandem ion source. Here results which focus on comparing operation in hydrogen and deuterium are presented. Several isotope effects are evident. In particular, for the same operating conditions, the electron density is found to be higher in deuterium than in hydrogen while the negative ion density is consistently lower.