Though often considered as one of the main driving processes of the complexation of species of opposite charges, the release of counterions has never been experimentally directly measured on polyelectrolyte/proteins complexes. We present here the first structural determination of such a release by small angle neutron scattering in complexes made of lysozyme, a positively charged protein and of polystyrene sulfonate, a negatively charged polyelectrolyte. Both components have the same neutron density length, so their scattering can be switched off simultaneously in an appropriate "matching" solvent; this enables determination of the spatial distribution of the single counterions within the complexes. The counterions (including the one subjected to Manning condensation) are expelled from the cores where the species are at electrostatic stoichiometry.