Determination of the optimum composition of a urethane-silica mixture, which would form crosslinked products, presented a significant problem in these studies. The unmodified silica was dosed at various ratios as compared to urethane elastomer: 2.5, 5.0, 7.5 and 10.0 phr. The optimum amount of the filler (precipitated silica) was found to be 5 phr and this amount of silica was subjected to modification with 3 and 5 weight parts of aminosilane U-15 or mercaptosilane A-189, per 100 weight parts Of SiO2.