Accurate prediction of corrosion initiation period and service life of reinforced concrete structures requires accurate determination of their chloride diffusion coefficient and binding capacity. Temperature and exposure duration affect these properties, but there is paucity of knowledge regarding the effects of long-term exposure to sub-zero temperatures. Hence, the effects of sub-zero temperature, T, and exposure duration, t, on the chloride diffusion decay index, m, and binding in ordinary Portland cement concrete are investigated. Non-steady natural diffusion tests are conducted at + 5, 0, −5 and −15 °C for 3, 6, 12 and 18 months, and at the end of each period, the total and free chloride concentration profiles are determined. The index m is found to vary over a wide range, it being directly proportional to T and inversely to t. It drastically deviates from the commonly assumed value of 0.2 in the literature. Chloride binding is found to obey the Freundlich isotherm. The coefficient α of the isotherm equals 1.0 ± 0.16 and is only slightly dependent on T or t while its exponent β varies from 0.178 to 0.502, without exhibiting a systematic relationship with either parameter. Binding is noticed to increase with drop in temperature for the first three months of exposure, but over longer exposure durations, no systematic relationship is observed.