The chemical equilibrium constant for the dissociation of protonated N-methyldiethanolamine (MDEA, CH3N(C2H5OH)(2)) in aqueous solutions is determined from electromotive force measurements at temperatures from 278 K to 368 K. Experimental results are reported and compared to literature values. Experimental results (on molality scale) are correlated by In K = [-819.7/(T/K)] - 79.474 + 10.9756 In(T/K).