The measurement of the free chloride concentration in the pore solution of cement paste or concrete is of interest for assessing the probability of corrosion of the steel reinforcements embedded in concrete. This determination is performed through leaching or pore-pressing methods or through embedded potentiometric sensors into the hardened cementitious material. The potentiometric determination of chloride in cement paste pore solutions is limited by hydroxide ion interference, due to the high alkalinity of such media. The potentiometric selectivity coefficients, k(Cl-,OH-), are determined for a chloride ionic selective electrode in alkaline solutions simulating the electrolyte present within the pore network of cement paste and concrete. This is done using a fixed interference methodology, with least-squares non-linear curve fitting for obtaining the selectivity coefficients together with other relevant electrode parameters. The limit of detection of the Cl(-) ISE, due to OH(-) interference, varies with pH. For the pore solution corresponding to an ordinary Portland cement paste or concrete, this limit can be set between 3 x 10(-3) m, and 7 x 10(-3) m, i.e., approximately below a chloride concentration value of 1 x 10(-2) molal. Taking into account these limits of detection the free Cl(-) concentrations able to depassivate the steel, can be adequately determined by potentiometric measurements with calibrated ISEs in the expressed pore solutions of Portland cement concretes, without significant influence of OH(-) interference. Nevertheless, this effect can prevent the adequate measurement of low free Cl(-) concentrations, below the corresponding limit of detection value. (C) 2009 Elsevier B.V. All rights reserved.