A hydrogen-electrode concentration cell was used to monitor pH in a study of morpholine ionization in aqueous solutions to temperatures of 290 degrees C and to ionic strengths of 1 m, maintained with either sodium trifluoromethanesulfonate or sodium chloride. The resulting hydrolysis quotients were combined with values from two previous potentiometric investigations of this reaction, as well as existing heat capacity and apparent molar volume data, to provide a global fitting equation. The computed thermodynamic parameters for the acid dissociation of the morpholinium ion at 25 degrees C and infinite dilution are log K = (-8.491 +/- 0.003); Delta H = (39 +/- 1) kJ.mol(-1); Delta S = (-31 +/- 1) J.K-1 mol(-1); Delta C-p = (40 +/- 7) J.K-1.mol(-1); and Delta V = (3 +/- 1) cm(3).mol(-1). At low ionic strengths, including values at infinite dilution, the log Q data were found to be near linear with respect to the reciprocal temperature in Kelvin over the measured temperature range of 0 degrees C to 300 degrees C.