The paper presents a study on the evolution of polymer properties during fine grinding in a vibrated bead mill. Two polymers having different initial properties were tested: poly(vinyl acetate) (PVA) and polystyrene (PS). They were ground with various operating conditions and the evolution of the size, the morphology, the molecular weight, the crystalline structure and the glass transition temperature was characterised. It was shown that the fragmentation rate and the fragment morphology depend on the material nature and on the operating parameters. The energy supplied by the mill is first essentially used for particle fragmentation. Then, when a limit size is reached, chains are cut at their extremities. It is thus possible to grind a polymer without damage if the experimental time is well controlled. The crystalline structure of poly(vinyl acetate) becomes amorphous during the first grinding minutes, and the amorphisation time does not depend on the operating conditions. Finally, no variation of the glass transition temperature was observed in this study.