ABSTRACT: Sour taste is influenced by pH and acids present in foods. It is not currently possible, however, to accurately predict and modify sour taste intensity in foods containing organic acids. The objective of this study was to investigate the roles of protonated (undissociated) organic acid species and hydrogen ions in evoking sour taste. Sour taste intensity increased linearly with hydrogen ion concentration (R2= 0.995), and with the concentration of protonated organic acid species at pH 3.5 (R2= 0.884), 4.0 (R2= 0.929), and 4.5 (R2= 0.975). The structures of organic acid molecules did not affect sour taste after adjusting for the effects of protonated organic acid species and hydrogen ions. Sour taste intensity was also linearly related to the total concentration of protonated organic acid species in fresh-pack dill pickles (R2= 0.957). This study showed that the sour taste of organic acids was directly related to the number of molecules with at least 1 protonated carboxyl group plus the hydrogen ions in solution. Conclusions from these results are that all protonated organic acids are equally sour on a molar basis, that all protonated species of a given organic acid are equally sour, and that hydrogen ions and protonated organic acids are approximately equal in sour taste on a molar basis. This study provides a new understanding of the chemical species that are able to elicit sour taste and reveals a basis for predicting sour taste intensity in the formulation of acidified foods.