The acid dissociation constants of H3PO4 and H2PO4- in 1 and 3 m NaCl were determined from measurements of the electromotive force (emf) of cells without a liquid junction at intervals of 10 K from 268.15 to 318.15 K. The hydrogen gas electrode and silver-silver chloride reference electrode were used. The dissociation constants pK1* and pK2* (referred to standard states in pure NaCl solutions) were derived from the corrected emf data by a modified Harned-Ehlers method. The degree of ionization of H3PO4 in the solutions of the first step was large; hence, the values Of m(H) needed in the calculation were estimated from literature data for HCl/NaCl solutions of equal ionic strength and chloride molality. Errors introduced by approximations were eliminated by extrapolation to the limit of the pure aqueous NaCl solvent. Standard deviations of the intercepts (pK*) at the six temperatures varied from 0.006 to 0.029. The dissociation constants in the temperature range T = 268.15 K to T = 318.15 K were fitted to equations of the form pK* = A/T + B + C In T, the parameters of which are listed.