Drug substance impurities are routinely monitored using HPLC. Because HPLC retention times can vary, uncertainty can arise as to whether a peak at a new retention time is a new impurity. When impurity standards are not available some method is needed to characterize the impurities on-line. This work sought to assess the ability of LC/MS/MS to generate characteristic impurity 'fingerprints', comprised of a precursor ion mass plus at least three product ion masses. MS/MS fingerprints of a drug substance, DuP 941, and three of its impurities were first generated using available standards. Experiments varying collision cell parameters showed that collision energy must be specified in order to reproducibly generate characteristic MS/MS fingerprints. MS/MS fingerprints were also generated on-line for seven impurities in the earliest safety lot of DuP 941. Several subsequent lots of DuP 941 were examined to see how well their impurity fingerprints matched those from the earlier lot. Fingerprint reproducibility was very good for all impurities examined, even down to 0.01 UV area percent for some impurities. MS/MS fingerprinting was able to distinguish two impurities from one another which were known to be positional isomers. It also permitted assignment of tentative structures to the drug impurities.