Research has suggested that people may use different qualitative cues or one quantitative cue (trace strength, activation) to attribute memories to their source. In an experiment, the diagnosticity of a qualitative cue (semantic features) was manipulated between participants: Distinctness was high when items from source 1 and items from source 2 belonged to different semantic categories, while it was low when both sources contained items from both categories. Depending on the study-test interval (48 hr or 10 min), trace strength of test items was either low or high for all participants. Thus, the quantitative cue was always diagnostic. Participants primarily used semantic cues in source attributions of new items. Also, source identification performance profited from semantic dissimilarity, not from high trace strength. The findings indicate that qualitative cues may play a more prominent role than quantitative cues in source monitoring.