Solid, multicomponent, multiphase catalysts, for example platinum supported on SiO2, play an important technological and economic role. The correlations between catalytic activity and the physical and chemical properties of these materials have been the subject of many investigations (see, for instance, Treacy, Thomas &. White, 1988). From the point of view of small-angle scattering (SAS), these catalysts are favorable objects of study: contrasts are high, and the important morphological features are of a size appropriate for SAS measurements.