Psychophysical judgments often depend on stimulus context. For example, sugar solutions are judged sweeter when a tasteless fruity aroma has been added. Response context also matters; adding a fruity aroma to sugar increases the rated sweetness when only sweetness is considered but not when fruitiness is judged as well. The interaction between stimulus context and response context has been explored more extensively in taste-odor mixtures than in taste-taste mixtures. To address this issue, subjects in the current study rated the sourness of citric acid mixed with quinine (bitter), sodium chloride (salty), and cyclamate (sweet) (stimulus context). In one condition, subjects rated sourness alone. In another, subjects rated both sourness and the other salient quality (bitterness, saltiness, or sweetness) (response context). Sourness ratings were most sensitive to response context for sour-salty mixtures (i.e., ratings of sourness alone exceeded ratings of sourness made simultaneously with saltiness) and least sensitive to context for the sour-sweet mixtures (sourness ratings made under the 2 conditions were essentially identical). Response-context effects for the sour-bitter mixture were nominally intermediate. The magnitudes of these context effects were related to judgments of qualitative similarity between citric acid and the other stimuli, consistent with prior findings. These types of context effects are relevant to the study of taste-taste mixture interactions and should provide insight into the perceptual similarities among the taste qualities.