Although the perceptual response to environmental odors can be quite variable, such variation has often been attributed to differences in individual sensitivity. An information-processing analysis of odor perception, however, treats both the reception and the subsequent evaluation of odor information as determinants of the perceptual response. Two experiments investigated whether a factor that influenced the evaluation stage affected the judgement of odor quality and the degree of adaptation to the odor. People were surveyed in order to measure their tacit perceptions of the healthfulness or hazardousness of nine common olfactory stimuli, and the instructional context influenced quality perception. In a second experiment subjects were exposed to an ambient odor under one of three different conditions, and odorant characterization influenced the degree of adaptation to the odor. Subjects who were led to believe the odor was a natural, healthy extract showed adaptation; those told that the odor was potentially hazardous showed apparent sensitization; while those told that the odor was a common olfactory test odorant showed a mixed pattern: some exhibited adaptation, whereas others showed sensitization. However, detection thresholds obtained before and after exposure showed adaptation effects that are characteristic of continuous exposure. These findings raise the possibility that cognitive factors may be modulating the overall sensory perception of odor exposure (i) for some individual who exhibit extreme sensitivity to odors and (ii) in situations where adaptation to environmental odors is expected but does not occur.