Metakaolin is a promising pozzolanic material which is currently used for producing high performance, high strength, lightweight cement. In this work, the effect of partially substituting cement with metakaolin on durability has been studied. Two types of cement (CEM I & II) were used with up to 20% substitution by metakaolin. Compressive strength and chloride permeability were determined at 28, 90, 180 and 360 days of hydration and carbonation depth was determined at different CO2 exposure periods. Chloride permeability for specimens treated underwater was also determined, and a correlation with electrical conductivity was established. Experimental results show that the addition of metakaolin in cement leads to improving strength and durability. For both types of cement, 10% substitution by metakaolin resulted in decreased chloride permeability and increased compressive strength. At higher substitution ratios chloride permeability further reduces, at the expense of compressive strength. Finally, very good correlation is observed between non-steady state diffusion coefficient and electrical conductivity and between non-steady state diffusion coefficient and the initially measured current.