Nanoporous powders are prepared in a single step by spray-drying mixtures of small inorganic and larger organic nanoparticles. The structure of these powders has been studied as a function of the mixture's composition using silica and polybromostyrene nanoparticles. Scanning electron microscopy reveals the presence of an increasing concentration of mesopores as the concentration of polybromostyrene increases. By coupling thermogravimetric analysis and ultra-small-angle X-ray scattering, the structure is quantitatively linked to the composition of the porous grains. Experimental USAXS intensities are compared to scattering models for the composite powders. It allows to demonstrate that (i) all mesopores are empty even in the center of the grains, (ii) part of the polymer remains in the micropores of the dried grains. A quantification of the fraction of micropores filled by residual polymer is presented. Such a synthesis procedure can be used to produce hydrophobic porous powders in a one-step process.