Assessed the sourness of 5 acids (citric, hydrochloric, phytic, succinic, and gluconolactone) at 4 concentrations of each acid and in 16 mixtures with citric acid, using the method of magnitude estimation. Ss were 10 18-24 yr old US Army enlisted men. Power functions fit to the sourness of unmixed acids were either added directly to predict mixture sourness, or both acids in the mixture were first converted to equivalent levels of citric acid and then exponentiated according to the appropriate power function. Summation of sourness functions accounted best for mixtures of citric acid with gluconolactone. Summation of acid equivalents was better for mixtures with phytic acid. Both models predicted mixtures with succinic and hydrochloric acids equally well. Mixtures with hydrochloric acid were synergistic. Results are discussed in terms of possible mechanisms underlying taste mixtures. (PsycINFO Database Record (c) 2006 APA, all rights reserved)