The acid dissociation equilibrium involving aqueous ethanolammonium ion in the reaction, HO(CH2)(2)NH3+ reversible arrow HO(CH2)(2)NH2 + H+, was measured potentiometrically with a hydrogen-electrode concentration cell from (0 to 290) degreesC in sodium trifluoromethanesulfonate (NaTr) solutions at ionic strengths of (0.03, 0.1, and 0.3) molal. The molal dissociation quotients and selected literature data at infinite dilution were fitted by an empirical equation with five adjustable parameters involving functions of temperature, solvent density, and ionic strength. This treatment yielded the following thermodynamic quantities at 25 degreesC and infinite dilution: log K-a = -9.48 +/- 0.03, DeltaH(a) = (51.3 +/- 1.5) kJ.mol(-1), DeltaS(a) = (-9 +/- 5) J.K-1.mol(-1), DeltaC(pa) = (8 +/-17) J.K-1.mol(-1).