Experiments were conducted to assess the relation between concentration, or pH, and the perceived sourness of 24 acids. The psychophysical functions for sourness conform to the power relation S = kCn which relates sensory intensity, S, to physical concentration, C. Averaged across the 24 acids, the exponent for sourness was 0.85 for both molar and percentage concentrations, and about −1.70 for pH concentration. The intercept, k, which is a measure of relative sourness, differed across acids. The particular measure used to designate the concentration of an acid markedly influenced its magnitude and rank order of sourness.