•A new method was developed for measuring the void fraction for two-phase flows in debris beds.•This technology, based on the use of a capacitance probe, was validated by comparison to a weighing method.•Integral tests have been performed subsequently on single-size debris beds made of 4mm and 8mm glass beads.•The data demonstrate the strong impact of interfacial friction phenomena. The modeling of pressure drop for two-phase flows through porous media is a key point to assess the coolability of debris beds resulting from nuclear severe accidents. Models involve several parameters which are non-linear functions of the void fraction, e.g. relative permeabilities. Their identification requires that experimental data include the measurement of void fraction. This paper presents a new technique developed to reach this objective. The method is based on the use of a capacitance probe and has been validated by comparison with a weighing method. The validation has shown that the accuracy is better than 10%. The measurement device has been implemented in the CALIDE facility, at IRSN, which has been designed to perform air–water flow through debris bed. Tests have been carried out with beds made of single size 4mm and 8mm beads. Measurements of pressure drop and average void fraction are reported in the paper, for air and water flow rates representative of flows that would result of either the reflooding of the damaged core or the cooling of corium debris in a stagnant pool of water. Finally, the pressure drop models used in severe accident simulation codes, based on generalizations of the single-phase Ergun law, have been assessed against the new data. It has been observed that generalized Ergun laws including an interfacial drag term accurately predict the pressure drop and the void fraction for flows with a zero net water velocity.