In the present study, Spanish-English bilinguals’ perceptual boundaries between voiced and voiceless stops (a /b/-/p/ continuum including pre-voiced, voiceless unaspirated, and voiceless aspirated tokens) are shown to be modulated by whether participants are “led to believe” they are classifying Spanish or English sounds. In Experiment 1, simultaneous Spanish-English bilinguals and beginner second-language learners of Spanish labeled the same acoustic continuum in two experimental sessions (Spanish mode, English mode), and both groups were found to display language-specific perceptual boundaries (or session effects). In Experiment 2, early bilinguals and late second-language learners of various levels of proficiency participated in a single session in which, in random order, they labeled nonwords that were designed to prime either Spanish or English language modes. Early bilinguals and relatively proficient second-language learners, but not less proficient learners, displayed mode-specific perceptual normalization criteria even in conditions of rapid, random mode switching. Along with similar ones, the experiments reported here demonstrate that bilinguals are able to exploit language-specific perceptual processes (or norms) when processing speech sounds, which entails some degree of separation between their sound systems.