A photodetachment technique has been used to measure the negative ion densities in both the driver and extraction regions of a tandem, multicusp ion source operating with H2and D2. The measurements have been made at electron densities of up to 2×1018m−3by using time resolved measurement techniques while operating the source in a pulsed mode. This also allows a study of the afterglow (post discharge). Of particular interest has been the comparison between operation in hydrogen and deuterium. Under the same operating conditions, the negative ion density in deuterium is consistently found to be about 75% of that in hydrogen. An increase in H−and D−densities in the early post discharge has been observed in both the extraction and driver regions.