Thermal and dynamic mechanical characteristics of injection-moulded discs of polypropylene (PP) filled with 40% by weight of magnesium and aluminium hydroxides have been studied by means of differential scanning calorimetry and dynamic mechanical thermal analysis, and have been related to the anisotropy and microstructure heterogeneity of the discs. The effect of filler type, particle morphology and surface coating has been analysed. The nucleation activity of filler particles on PP has been quantified and found to be reduced in coated grades of magnesium hydroxide. The employed coatings worked isolating and preserving particles surface from direct interaction with polymer chains. The different orientations of both filler particles and PPalpha. crystals were found to be the main cause of the differences observed in mechanical properties.