Polypropylene (PP)-clay composites were prepared by melt mixing in an intensive mixer. Three grades of PPs with different melt viscosities were employed to investigate mixing characteristics and phase structure of the composites with various clays that belong to organically modified montmorillonite (org-MAM. Depending on the matrix viscosity and nature) of the organic layer in MMT, significant variations in the phase structure of the composites were found. In addition to the simple combination of PP and clay, various functionalized PPs were also incorporated in an attempt to enhance thermodynamic interaction between the org-MMT and PP matrix. Major interest was focused on the effect of varying thermodynamic affinity between the components on the phase evolution and mechanical properties of the composites. Within the available range of maleic anhydride (MA) content in PP, it was found that optimum content of the functional group exists, which balances favorable interaction with org-MMT and tolerable homogeneity with PP matrix. Along with keeping the optimum MA content, it was also important to use a low viscosity PP matrix to achieve a more random array of layered silicates. The observed phase structure was interpreted by using a model based on self-consistent field theory.