The free chloride and hydroxyl ion concentrations of the pore electrolyte phase present in mature, hydrated cement pastes containing equivalent quantities of chloride ion, introduced into the mix water as NaCl or CaCl//2, have been determined for two commercial cements. The corrosion rates of mild steel electrodes embedded in these materials have also been monitored by linear polarization. Specimens containing CaCl//2 are more aggressive than those with NaCl but the effect appears to be a result of changes occurring in the early stages of hydration rather than of longer term modifications in the pore solution chemistry. The two cememts were used a typical British Ordinary Portland cement (OPC) and Spanish Portland blast furnace cement (BFSC) made by intergrinding Portland cement clinker and blast furnace slag.