Concrete pore solutions are concentrated electrolytes in which the activity of the chemical species must be calculated properly to determine the equilibrium state of such solutions. In this work, the chloride-ion activity coefficients in synthetic con crete pore solutions have been obtained potentiometrically, using different calibrated Cl- ion-selective electrodes (ISEs); also, a discussion about the performance of the different Cl- ISEs in alkaline media is presented. Experimental results are compared with the calculated results, using Fitter's specific ionic-interaction model, and good agreement is observed between them.