The concept of liquid-phase dispersion was applied for the preparation of well-dispersed suspensions of MWCNTs and graphene in chloroform, using long-time ultra-sonication without the use of surfactants. The dispersions with pre-defined filler concentration (0.5 mg/ml) were monitored via UV–Vis until the achievement of optimum exfoliation (6 h). PS/PPO polymer was dissolved in chloroform and graphene- and MWCNTs-based composite tablets were prepared via solution blending, subsequent drying and hot pressing. The well-dispersed organization of the fillers inside the polymer matrix, visualized with scanning electron microscopy, resulted in ultimate conductivities and percolation thresholds of 57 S/m and 0.2 wt.% for nanotubes composites, and 0.9 S/m and ~1 wt.% for graphene composites, respectively. Dynamic mechanical analysis showed that an increase in the storage moduli of the PS/PPO matrix could be gradually obtained by the insertion of fillers, e.g. reaching ~30% of enhancement by the addition of 3 wt.% of graphene filler. The same trend in improvement, at lower augmentation, was observed for the corresponding nanotubes-based composites.