Subliminal perception occurs when prime stimuli that participants claim not to be aware of nevertheless influence subsequent processing of a target. This claim, however, critically depends on correct methods to assess prime awareness. Typically, d' ("d prime") tasks administered after a priming task are used to establish that people are unable to discriminate between different primes. Here, we show that such d' tasks are influenced by the nature of the target, by attentional factors, and by the delay between stimulus presentation and response. Our results suggest that the standard d' task is not a straightforward measure of prime visibility. We discuss the implications of our findings for subliminal perception research.