Seven experiments investigated how stimulus context affects judgements of the magnitude of chemosensory stimuli. In each experiment, subjects gave magnitude estimates of the intensity of several concentrations of two substances, with the contextual set of concentrations varying across experimental conditions. Different experiments used different pairs of substances, which could be tastants (sucrose or NaCl) that were sipped, odorants (orange or vanillin) that were sipped (i.e. presented retronasally) or the same odorants sniffed (i.e. presented orthonasally). Varying the stimulus context affected the judgements differentially when the two substances were compositionally different (sucrose and NaCl; sucrose and orange; sucrose and vanillin) but not when they were the same (vanillin or orange presented orally and nasally). Judgements of qualitative similarity of the same pairs of substances, obtained in a separate experiment, failed to predict accurately the pattern of differential context effects. Taken together, the results suggest that differential effects of context relate only indirectly to perceptual dissimilarity per se but may primarily reflect the result of stimulus-specific adaptation-like processes.