Mixed aggregates of a short-chain nonionic amphiphilic poly(p-phenylene), in which each benzene ring of the aromatic backbone is 2,5-disubstituted with a hydrophobic and a hydrophilic group, and of the nonionic surfactant C8E4, C12E6, or C(10)G(2) were studied in aqueous solutions. Cryo-transmission electron microscopy and light scattering experiments revealed the existence of fiberlike aggregates of mean length greater 200 nm and a diameter of ca. 6 nm in an excess of small surfactant micelles in these solutions. In the present contribution small-angle neutron scattering (SANS) is used to characterize the fiber aggregates in these complex liquids with respect to their composition and their radial scattering length density distribution sigma(r). The composition of the fiber aggregates (polymer-to-surfactant number ratio) was determined using perdeuterated surfactant and contrast variation of the solvent (H2O/D2O)Moreover, the diameter of the mixed aggregates is determined using form factors for descriptions of the scattering curves and additionally numerical approaches to treat the data. The ratio of surfactant molecules to PPP was found to be 3:1 (20 wt %:80 wt %).