The void space in a variety of objects can be obtained by a neutron activation method utilizing the normal argon content in the air filling the pores and voids of such samples. The activation reaction used is40Ar(n,γ)41Ar which has a cross section of 0.63 b. Argon-41 decays by beta emission with a half-life of 1.85 h to the stable isotope of potassium,41K.If the quantity of air in a sealed container is known, the air can be used as an activation“foil”to estimate the total flux seen by the sample while in the reactor. After standard air volumes have been evaluated, porous solids containing air can be analyzed for void space if placed under the same conditions of geometry and efficiency and the activity acquired used as a measure of the total air present relative to a standard. It is possible to use this technique with substances for which the porosity (void space/bulk volume) is difficult to measure in other ways.