We discuss the effects of the melt compounding variables, matrix molecular weight and organoclay content on the X-ray diffraction (XRD) and mechanical properties of polystyrene (PS)/organoclay nanocomposites (PNC) prepared in a twin-screw extruder. An increase of residence time reduced the height of the first XRD peak and increased that of the second peak. Barrel temperature and screw configuration had a little influence on the tensile properties and impact strength. Young's modulus increased with organoclay content and it was almost independent of matrix PS molecular weight. The stress-at-break and impact strength decreased with organoclay content and increased with PS molecular weight. Young's modulus and impact strength decreased with residence time. Since the slopes of these dependencies for PNC were similar to that of the neat PS, the matrix degradation seems to play the major role. The relationship between impact strength and elongation at break of PNC showed high dependency on matrix grade. However, better empirical correlation was observed between the impact and tensile strengths. According to theoretical model of Ji et al. [32], the Young's modulus vs. clay concentration dependence indicated the presence of low interphase thickness. (c) 2005 Society of Plastics Engineers.