Abstract. Polypropylene (PP)/organophilized montmorillonite (OMMT) and polypropylene/organophilized montmorillonite/maleic anhydride grafted polypropylene (MAPP) composites were prepared in an internal mixer under a wide range of processing conditions to study the kinetics of structure formation. Structure and properties were characterized by a variety of techniques. The gallery structure of the organophilic silicate changed in spite of the fact that no compatibilizer was added to PP/OMMT composites. Silicate reflection shifted towards smaller 2θ angles, broadened and its intensity decreased indicating intercalation. Transmission electron microscopy (TEM) micrographs even showed individual platelets at long mixing times. However, the extent and direction of changes in the gallery structure of the silicate did not justify those observed in properties. The analysis of the results and additional experiments proved that the degradation of the polymer also takes place during processing leading to the formation of carbonyl and/or carboxyl groups, as well as to the decrease of molecular weight. The modification of the chain structure of the polymer influences interfacial interactions and the intercalation process. Some properties are directly determined by molecular weight (rheological properties, elongation). Both the clay and the MAPP seem to accelerate degradation. Thermooxidative degradation must have disadvantageous effect during the application of PP nanocomposites and needs further study. : nanocomposites, layered silicate, polypropylene, gallery structure, thermal degradation eXPRESS Polymer Letters Vol.3, No.11 (2009) [692][693][694][695][696][697][698][699][700][701][702] Available online at www.expresspolymlett.com DOI: 10.3144/expresspolymlett.2009.87 complicated systems containing several components like the polymer, the silicate, the surfactant, and quite often also swelling and coupling agents or other additives to help exfoliation [7][8][9][10][11][12][13][14]. The competitive interactions acting among the various groups of the components are very difficult to estimate and control, which results in a wide range of structures often with inferior properties. Similarly to the uncertainties related to the role and effect of interactions on the structure and properties of nanocomposites, the effect of kinetics, i.e. processing conditions, on structure development is also quite unclear. Some groups claim that processing conditions, or even preparation conditions do not influence the structure of the composites [15,16], while others found a strong effect of processing [17,18]. For example Yilmazer and Ozden [16] compared the effect of three preparation conditions, i.e. in situ polymerization, melt compounding and a masterbatch method, and did not find any significant difference in structure and properties of polystyrene (PS) nanocomposites. On the other hand, Paul and co-workers [19][20][21] carried out a detailed study on polyamide 6 (PA6) nanocomposites using a single and three twin-screw extruders...