Pore size distribution, porosities and consequently derived densities are key parameters for the qualification of inert matrix fuel (IMF). Porous features must be determined accurately for assessing the in-pile behaviour of the fuel. The analytical methodologies to measure porosities are revisited. The paper discusses in a comprehensive way the results obtained for simulated or plutonium loaded inert matrix fuel i.e. erbia and ceria or plutonia doped yttria stabilised zirconia, by applying thermodynamic or instrumental approaches. The n-D results (open/close porosities) gained by applying invasive/non-invasive, microscopic/macroscopic, neutron/photon and reflection/diffraction/transmission methods with fluid intrusion or not, are given and discussed for the zirconia based cubic solid solution.