Measurements have been made of the solubility at 25°C. of tyrosine in hydrochloric acid and in sodium hydroxide solutions varying from 0.001 to 0.05 M, and also in distilled water. The pH of the saturated solutions was measured with the hydrogen electrode. The following values for the ionization constants of tyrosine have been obtained from the measurements: kb = 1.57 x 10–12, ka1 = 7.8 x 10–10, ka2 = 8.5 x 10–11. The changes in solubility with pH can be satisfactorily explained by the use of these ionization constants.