An experimental method based on a falling pressure measurement is proposed to evaluate the through thickness air permeability of prepregs during cure, with two set-up variants. One concerns the measurement of the inherent air permeability and the other applies to processing conditions, implying the use of consumables and vacuum-bag. The methods were successfully tested and fulfil the criterions used in soil science for the evaluation of the air permeability. Prepreg permeability to air during cure varies between 10−17 m2 and 10−19 m2 and is found to depend on the evolution of the resin viscosity, which shifts from an initially immobile phase to a mobile one and back to an immobile phase.