The organisation of cationic surfactants (DPCl and CPCl) adsorbed in the cylindrical pores of a periodic mesoporous silica (SBA-15, pore diameter ca. 8 nm) has been studied by small-angle neutron scattering at different solvent contrast conditions. In the experimental
range the scattering curve of SBA-15 is dominated by Bragg peaks resulting from the ordered array of mesopores. It is found that the peak intensities are strongly affected by the surfactant adsorption and the solvent contrast. The scattering curves can be modelled by combining the structure factor of a 2D hexagonal lattice with the form factor of a core-shell cylinder, in which the adsorbed surfactant film is represented by the shell of the cylinder. This simple model accounts for the main experimental findings and provides a direct method for estimating the thickness of the adsorbed film in the pores.