The properties of different definitions of the cross section for helium scattering of perfectly diffuse adsorbates on highly corrugated surfaces have been investigated experimentally. For the adsorption of CO on Rh(311) we found that the normalized cross section associated with each diffraction beam scatters strongly with the angle of incidence, the incident energy, and the intensity of the diffraction beam. In particular, our results demonstrate that, contrary to the suggestion of a recent work, no simple relation exist between the intensity of a given diffraction beam and the associated cross section. On the other hand, a total cross section defined by including all diffraction beams was found to vary smoothly as a function of incident parameters, in a way similar to that observed on low corrugated surfaces and in gas phase scattering. (C) 2000 American Institute of Physics. [S0021-9606(00)70410-7].