The effects of diffraction on capture of the radiation into narrow channels between two weakly rough surfaces pressed together ('slitless collimator') and channeling of x-radiation in a three-layered structure with low atomic number thin film inside were investigated, The influence of channel shape distortion and its surface quality on the formation of the outgoing beam was investigated. A plausible explanation is given for the observed effect of abnormality of transmission through Cr/C/Cr thin-film channels with account of periodic deformation, The calculations of the X-ray waveguide mode attenuation in thin rough channels were compared with the results of numerical integration of the quasi-optical equation for electrical field amplitude. Copyright (C) 2002 John Wiley Sons, Ltd.