Wide- and small-angle X-ray scattering (WAXS and SAXS) and X-ray microtomography were used to study the structure of brittle, solid foams made by extrusion of whole grain barley flour and additional whey protein isolate (WPI) and polydextrose (PD). The structure of the extrudates was described in a coarse way from nano- to microscale, and the effects of additives were discussed. The additives were observed to affect the structure in many scales, for instance, by inducing differences in the crystal structure of starch and in the microscale pore structure observed with both SAXS and X-ray microtomography. The most significant effects were introduced by WPI, but these were highly reduced in the presence of PD. Differences were also seen in lamellar structures of starch, which were probably formed due to retrogradation and observed with SAXS in powder samples soaked in water before the measurements.