The influence of low load reinforcing fillers in carboxylated nitrile rubber (XNBR), crosslinked with magnesium oxide, has been studied. The nanofillers utilized were carbon black, precipitated silica, natural clay, and synthetic silica. The latter filler was obtained by acid lixiviation of a natural magnesium silicate. The rheological behavior of the samples was study by means of capillary (consistency index) and rotational (Mooney viscosity) flows. Through these analyses, higher viscosities, and lower flow indexes were observed for the filled samples. The acid character of silica and clay leads to an increase in vulcanization rates, measured by the cure rate index (CRI) and referring it to the unfilled compound. Moreover, the filters dilute the ionic microphases formed during the vulcanization process of the XNBR. Since these microphases behave as reinforcing sites, a decrease in tear and abrasion properties takes place. However, the reinforcing effect of the fillers leads to higher values of stress at constant strains. Finally, the variation of glass and ionic transition temperatures with filler addition was analyzed. (C) 2007 Wiley Periodicals, Inc.