This International Standard describes the principles and methodologies of measuring both gas open porosity andpermeability of hydraulic binder-based matrices used for immobilization of radioactive waste. The measurementscan be carried out by using different apparatus designed and constructed on the basis of a few recommendedcharacteristics. The measurements can be performed on samples prepared in a laboratory or taken from industrialproduction. Samples can be obtained by moulding or by coring a block.