The second dissociation constant pK(2) values for 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) and 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO), useful secondary standard buffers within the pH range of 9.7 to 10.5 for CAPS, and 8.9 to 10.0 for CAPSO, have been determined from T = 278.15 K to T = 328.15 K. Electromotive force measurements of cells of the type : PtH-2 (g,p degrees = 101.325 kPa)buffer (m(1)), Na salt of buffer (m(2)), NaCl (m(3))AgClAg, where m denotes molality, were used. The pK(2) for the ionization of CAPS in the experimental temperature range is expressed by the equation : pK(2) = 1046.64.(K/T) + 35.10191 - 4.9342.ln(T/K), whereas that for CAPSO is given by : pK(2) = 2106.21.(K/T) + 9.08772 - 1.11049.ln(T/K). The thermodynamic quantities have been calculated and the results obtained are compared with those for zwitterionic compounds which are the derivatives of taurine.