The study of chemical and electrochemical equilibria between concrete or embedded steel and ionic species in concrete's pore solution requires the knowledge of ion activities (free concentrations in solution). Recently, a method has been developed for measuring chloride-ion (Cl-) activities in simplified synthetic concrete pore solutions using calibrated Cl- selective electrodes. Cl- activities also have been calculated using Pitzer's specific ionic-interaction model. In this work, we have applied both of these methods, experimental and theoretical, for obtaining Cl- activities in solutions obtained using the technique of "pore pressing," which has been applied to cement pastes with varied admixed Cl- concentrations. A reasonably good agreement has been found between experimental and theoretical results, but the presence of hydroxide ions in these highly alkaline electrolytes interferes with the measurements at low Cl- concentrations.