Composites of poly(vinyl acetate) filled with calcium carbonate (CaCO3) have been synthesized in a ball mill. The different steps in fragmentation and agglomeration phenomena have been identified. The materials have first been ground separately to characterize their behaviour in the mill. Then, they were ground together. It has been shown that small CaCO3 fragments cover PVA particles, limiting for a first time the agglomeration of these and permitting their size reduction. Then, when PVA fragments are small enough, a competition between size reduction and agglomeration occurs, which favours the dispersion of CaCO3 in the matrix. It has also been observed that the presence of the filler does not modify significantly the molecular mobility of PVA.