The acid dissociation equilibria involving aqueous dimethylammonium ion in the reaction (CH3)2NH2+ ⇄ (CH3)2NH + H+ were measured potentiometrically with a hydrogen-electrode concentration cell from (0 to 290) °C in sodium trifluoromethanesulfonate (NaTr) solutions at ionic strengths of (0.1, 0.3, and 1) molal. The molal dissociation quotients and selected literature data at infinite dilution were fitted by an empirical equation involving six adjustable parameters involving functions of temperature, solvent density, and ionic strength. This treatment yielded the following thermodynamic quantities at 25 °C and infinite dilution: log Kd = −10.77 ± 0.02, ΔHd = (50.8 ± 0.7) kJ·mol-1, ΔSd = (−35.8 ± 2.5) J·K-1·mol-1, ΔCp,d = (116 ± 11) J·K-1·mol-1, and ΔVd = (−4.3 ± 2.5) cm3·mol-1.