Abstract: An apparatus for testing the relative permeability of materials, particularly concrete structures, comprises a transparent, optically non-distorting head having a gasket, which together with the head defines a head chamber, and which allows the head to be sealed against the structure when a partial vacuum is induced in the head chamber. The decay of the vacuum may be measured, and the rate of decay provides an index of permeability. A liquid may be applied to the surface being tested and will foan to allow identification of cracks and fissures. In alternative embodiment, a fluid is introduced into a secondary chamber defined by a secondary gasket and the head, and the rate of permeation of the fluid into the structure provides a measure of permeability.