Study of physisorption by neutron diffraction is discussed critically, especially in relation to Our previous results for krypton or graphite. It is shown that the position of the superstructure peak gives little information ; but its low intensity can be related to existence of antiphases on a scale smaller than 100 Å. Variations of graphite peaks are difficult to conciliate with adsorption isotherms : they imply persistence of registry even in the bilayer. Means to overcome difficulties coming from stacking faults in the substrate are indicated