Blends of 75/25 chloroprene rubber(CR)/natural rubber (NR) filled with various loadings of precipitated silica were prepared and their processability and mechanical properties as well as their resistance to thermal aging and oil were determined. The blend morphology was also studied using the atomic force microscopy technique. The results reveal that the mixing energy and the Mooney viscosity of the compound are increased continuously with increasing silica loading. It is also found that both scorch and optimum curing times are shortened while the total crosslink density is increased with increasing silica loading. The positive effect on cure could be explained by the chemical reaction between the allylic chlorine atom of CR and the silanol group on silica surface. The tensile strength, modulus, and hardness of the blend vulcanizate are noticeably improved while the compression set at elevated temperature is impaired with increasing silica loading. The results also reveal that both thermal aging resistance and oil resistance of the blend vulcanizates, as represented by the relative properties, are enhanced with the addition of silica. The resistance enhancement is believed to arise from the combination of the dilution effect, the increased crosslink density and also the reduction of NR dispersed phase size. (c) 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3478-3483, 2007.