The language of regulations implementing the 1977 Clean Air Act Amendments distinguishes between humanly detectable visibility impairment in Class I Parks and Wilderness areas and the extent to which such impairment adversely affects a park visitor's visual experience. Two lines of environmental perception research have arisen from this distinction, one emphasizing the detectability of visibility impairment, and the other emphasizing a park visitor's experience. This study attempts to deal with issues relevant to both lines of research. Subjects were shown color slides depicting varying levels of visibility in Class I Parks and Wilderness areas under four treatment conditions. A sensitivity gradient emerged from these conditions: Subjects who rated repeated versions of selected vistas for visual air quality were most sensitive to changes in visibility, while subjects who rated a random series of scenes for scenic beauty were least sensitive. Other variables (i.e., scattering angle, and the particular vista being viewed) also proved to have substantial influence on perceptual ratings.