The taste of most fruits is characterized by a mixture of sensations termed sweet and sour by humans, and the food selection behavior of primates suggests that they may use the relative salience of sweetness and sourness to assess palatability of potential food items. Therefore, taste responses of six squirrel monkeys, five pigtail macaques, four olive baboons, and four spider monkeys to sweet-sour taste mixtures were assessed in two-bottle preference tests of brief duration (2 min). Monkeys were given the choice between a reference solution of 50 mM sucrose and mixtures containing 10, 30, or 50 mM citric acid plus 10, 20, 50, 100, 200, 400, 800, or 1000 mM sucrose. We found that the four species differed markedly in their acceptance of physiological concentrations of sour-tasting citric acid. Whereas olive baboons showed the highest degree of sour-taste tolerance and actually preferred most of the sweet-sour taste mixtures over sweet-tasting reference solutions, squirrel monkeys showed the lowest degree of sour-taste tolerance and rejected most of the sweet-sour taste mixtures even when they contained considerably more sucrose than the reference solutions. Additional tests demonstrated that the preference for sweet-sour taste mixtures was not based on masking effects. Rather, the animals perceived both the sweetness and the sourness of the taste mixtures and made a trade-off between the attractive and aversive properties of the two taste qualities. The results of this study suggest that the proximate reason for the marked differences in acceptance of sweet-sour taste mixtures are differences among species in the hedonic evaluation of the sour taste of citric acid. Possible ultimate reasons, which do not necessarily exclude, but may complement each other, include evolutionary adaptation to dietary specialization, avoidance of competition pressure, and phylogenetic relatedness.