The effect of magnesium carbonate (MgCO3) loading on the compound properties of polychloroprene (CR), natural rubber (NR), and their blends was investigated. The studied properties included Mooney viscosity, cure characteristics, and degree of filler-filler interaction. In addition to MgCO3 loading, the effect of the blend ratio on such properties was simultaneously determined. The results reveal that, regardless of the rubber type and blend ratio, increasing MgCO3 loading resulted in not only an increase in the compound viscosity, but also in the enhancements of the cure rate and state of cure. However, the effect of MgCO3 loading on the overcured behavior was strongly dependent on the rubber type and blend ratio. For CR, the presence of MgCO3 accelerated postcuring, whereas the opposite results were found for NR, in which the reversion rate was promoted. For CR/NR blends, a plateau behavior was obtained at a 50/50 blend ratio, where the degrees of postcuring and reversion were counterbalanced. The results also reveal that when MgCO3 was loaded at low concentrations (<= 20 phr), the degree of filler-filler interaction was very low and had no significant effect on the compound properties. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2763-2769, 2008