Although correlations between the final dispersion level of organoclay nanocomposites and melt mixing conditions in a twin screw extruder have been proposed, the actual lengthwise evolution of dispersion in these machines is still not well understood. Despite their practical importance such studies are difficult, due to the possibility of inducing degradation and/or morphology changes during material collection and/or sample preparation. This work uses on-line rheometry and in-line near-infrared spectroscopy (NIR) to monitor the evolution of dispersion of a PP/PP-g-MA/organoclay system along the axis of twin screw extruder. Regardless of the screw profile utilised, dispersion develops quickly in the first part of the machine, and then it levels-off or decreases, even if the final dispersion levels are significant. In order to validate this data, Scanning Transmitted Electron Microscopy (STEM), X-ray diffraction (XRD) and mid-FT-IR analyses are carried out on samples collected from the same locations where the on-line/in-line measurements were performed. The results confirmed the previous measurements. Thermogravimetric analysis (TGA) and Time of Flight-Secondary Ion Mass Spectrometry (TOF-SIMS) on clay, matrix and nanocomposite showed that thermal degradation of the clay surfactant and of the polymer matrix could contribute to the observed dispersion reversal. (c) 2013 Elsevier Ltd. All rights reserved.