The precise electromotive force (emf) measurements of the cells without liquid junction, PtH-2ACES (m(1)) + NaACESate (m(2)) + NaCl (m(3))AgCl, Ag and PtH-2CHES (m(1)) + NaCHESate (m(2)) + NaCl (m(3))AgCl, Ag, have been made to collect emf data from (5 to 55) degrees C, with molalities m(1) = m(2) = m(3) in the range 0.01 to 0.1 mol . kg(-1). The hydrogen and silver + silver chloride electrodes were used. The second thermodynamic dissociation constants (pK(2)) of N-(2-acetamido)-2-aminoethanesulfonic acid (ACES) and 2-(N-cyclohexylamino)ethanesulfonic acid (CHES), along with the thermodynamic quantities associated with the ionization of ACES and CHES, have been calculated. The values of pK(2) for AGES are expressed by the equation pK(2) = 2548.60/(T/K) - 20.03159 + 3.2172 ln(T/K), and for CHES, by pK(2) = 1917.66/(T/K) + 5.7940 - 0.4971 ln(T/K) with standard deviations of +/-0.000 58 and +/- 0.000 97, respectively.