Salts containing divalent tin (SnII) are shown to give a measure of corrosion protection to mild steel embedded in cement paste. Results of linear polarisation measurements are shown for samples cured at 40-degrees for up to 240d in pastes containing initially 0-1628mM/l chloride: Sn2+, but not Sn4+, is an effective inhibitor at an initial concentration of 200mM/l. However tin solubility pore fluid is very low: less than 0.01mM/l at 60d. It is speculated that tin stabilises the passivating layers although no direct evidence for the incorporation of tin in corrosion product was found by analytical electron microscopy. Other theories of inhibition arc discussed.