Polymer composites of several concentrations of poly(methyl methacrylate) and poly(p-phenyleneterephthalamide) microfibrils have been prepared by blending a solution of the matrix polymer and a suspension of microfibrils obtained from a solution of commercial Kevlar fiber in H2SO4. Scanning electron microscopy shows that a continuous network of fibrils with dimensions in the range 30-70 nm builds up starting at very low PPTA concentrations and that these fibrils tend to form larger globular aggregates at higher PPTA concentrations. The high connectivity of the network is reflected in the good reinforcing effect as shown by the values of the dynamic-mechanical storage modulus. Increased interaction between the constituent phases as the fiber content grows is revealed by an increase of the calorimetric glass transition temperature and by thermogravimetric analysis, as well as by the growth of the slope of the Arrhenius plot of the main relaxation. The induced modifications of the matrix polymer’s dynamic-mechanical spectrum are discussed with the help of predictions of a modified block model. A consistent interpretation of the temperature shifts of the glass transition and the main dynamic-mechanical dispersion can be gained on this basis. The numerical values of the model parameters that fit the experimental behavior correlate well with the buildup of structure revealed in the micrographs.