Through this research, it was concluded that hydrogen ions are the fundamental substance producing a sour taste. Hydrogen ions have two forms: one is free hydrogen ion species, and the other is the undissociated hydrogen species, which is bound to the anionic acid species. The intensity of sour taste is related to the total con- centration of free and undissociated hydrogen ions. The threshold of sour taste of pure free hydrogen was determined to be 0.575 mM, which corresponds to pH 3.24. The anionic acid species has no sour taste, but can affect the intensity of sour taste of the hydrogen ion which is bound to it. The effects of anionic acid spe- cies on the intensity of the sour taste are determined by structure. The effects will be similar if the structures of the anionic acid spe- cies are similar. The hydroxyl or conjugated groups in the anionic acid species can intensify a sour taste, so the thresholds of organic acids which contain these groups will be lowered.