A non-destructive, rapid test capable of measuring the air permeability of in situ concrete is described. The technique is based on the application of a hard vacuum to the concrete surface. The rate of pressure recovery is recorded after the vacuum line is disconnected. The experimental results lead to an exponential pressure–time relationship similar to that obtained from analytical solution of Darcy’s equation. The air permeability of concrete can be simply expressed in terms of a vacuum decay parameter, but minor changes in the application of the test make it possible to determine the air permeability of concrete in situ. The test results are sensitive to changes in the parameters that affect the pore structure of concrete, including water–cement ratio, curing conditions, cement content, air content and plasticizing admixtures.