Electromotive-force measurements were taken for cells without a liquid junction of the type : Pt \ H-2(g, p(0) = 101.325 kPa) \ buffer (m(1)), Na salt of buffer (m(2)), NaCl (m(3)) \ AgCL \ Ag, where m denotes molality. Values of the second thermodynamic dissociation constant pK(2) for N-(2-hydroxyethyl)piperazine-N＇-3-propanesulfonic acid (HEPPS) and (3-[N-morpholinol])-2-hydroxypropanesulfonic acid (MOPSO) have been determined from T = 278.15 K to T = 328.15 K. These zwitterionic compounds are useful as secondary standard buffers in the pH range 7.6 to 8.2 for HEPPS, and 7.2 to 6.4 for MOPSO. The pK(2) values for the ionization of HEPPS in the experimental temperature range are expressed by the equation : pK(2) = 359.23.(K/T) + 21.0324 - 2.50647.ln(T/K); while, for MOPSO, pK(2) = 432.25.(K/T) + 22.99923 - 3.08415.ln(T/K). The values of the thermodynamic quantities are compared with those for structurally related compounds.