We study for the first time the use of pass/fail fault dictionaries for fault diagnosis of circuits with scan. The special structure of the tests generated for scan circuits allows us to use pass/fail information for primary output sequences, for scan-out vectors, or for both. We present experimental results to demonstrate the numbers of fault pairs that can be distinguished by the different types of pass/fail dictionaries, and the dictionary sizes. We also describe a dictionary enhancement method that allows all the fault pairs distinguishable by a full fault dictionary to be distinguished by the enhanced pass/fail dictionary. We use two sets of scan-based tests for the experiments we conduct, and we demonstrate the benefits of test sets containing tests with longer sequences of primary input vectors.