Carbonation is widely recognized as a significant cause of corrosion of reinforcement in concrete. However, in addition to the depassivation of the steel, it has several other effects. This paper investigates the effect of carbonation on the permeability and pore structure of concrete. One of the most significant consequences of this effect is that it causes standard tests for durability to give misleading results. Suitable test procedures for the measurement of porosity and permeability were chosen form a review of the available techniques. Samples of two concrete mixes with different water-cement ratios were each prepared with both wet and dry curing and exposed in a carbonation chamber for up to 140 days. The results show that carbonation reduces the permeability and porosity and indicate the extent of this reduction. The reduction was greatest in poor quality concrete. When compared with the other effects of carbonation, the reduction in permeability is seen to be similar to the reduction in electrical resistivity because this can also lead to misleading test results.