The rheological properties of polystyrene (PS) compounds containing cross-linked PMMA particles are characterized by oscillatory shear experiments, while the amount of covalently grafted carboxylic-acid terminated polystyrene (CT-PS) on their surface is varied. All samples show an additional relaxation process with a long relaxation time in the frequency range where the matrix hows. The strength of this process depends strongly on the amount of particles, i.e., on the degree of filling and, to the same extent, on the different "coverage" of the particles surface, i.e., the degree of grafting. For the highest degrees of filling and in dependence on the degree of grafting a particle network is formed which is characterized by an evolving equilibrium modulus. Moreover, compatibilization by grafting CT-PS onto the PMMA particles' surface reduces the strength of the additional relaxation process remarkably. This surprising effect is related qualitatively to the balance of the interaction between particles and the interaction between the particles and the matrix due to PS grafts. Some of these results can be understood quantitatively on the basis of the sticky hard-sphere model.