Synopsis A permeability cell for measuring the permeability of mortar and concrete to gas is described. The problems encountered and overcome during the development of the cell are discussed. The cell is designed to test 25 mm diameter mortar samples but can also be used for 50mm diameter concrete specimens. Specimens of heights between 10 and 50mm can be tested. Measurements using the new cell are highly repeatable and reproducible. The cell is very sensitive to the influence of water/cement ratio, age and replacement of cement by pulverized-fuel ash. It can be used to obtain permeability profiles through the depth of a concrete element in order to evaluate the influence of different curing conditions or curing compounds. Synopsis A permeability cell for measuring the permeability of mortar and concrete to gas is described. The problems encountered and overcome during the development of the cell are discussed. The cell is designed to test 25 mm diameter mortar samples but can also be used for 50mm diameter concrete specimens. Specimens of heights between 10 and 50mm can be tested. Measurements using the new cell are highly repeatable and reproducible. The cell is very sensitive to the influence of water/cement ratio, age and replacement of cement by pulverized-fuel ash. It can be used to obtain permeability profiles through the depth of a concrete element in order to evaluate the influence of different curing conditions or curing compounds.