Using chemically different substances as co-adsorbates allows one to block certain types of adsorption sites in a given porous material or even on a surface. This can be very useful for distinguishing and identifying various different adsorption sites in an heterogenous adsorbent. It has also the potential of helping in better and more reliable interpretation of inelastic neutron scattering spectra of adsorbed dihydrogen for instance. Particularly useful the technique can be where different set of sites with very similar adsortpion heats are present or different types dihydrogen complexes on a certen complex forming centre may be present. In the case of the CPO-27-Fe, preadsorption of O2 can be used to poison the "metal" sites located just above the Fe(II) ion by forming a stable Fe(II)-O2 oxocomplex, then dose H2, and re-measure an INS spectrum free of Fe(II) sites contribution.