The dynamic viscoelasticity was investigated for isoprene rubber (IR) composites filled with untreated and chemically treated cellulose fibers. Mercerization and benzylation were employed as chemical treatments. Bleached kraft pulp and cellulose powder were used as cellulose fillers. By mixing cellulose fillers, the dynamic modulus E' of IR composites was improved, especially above the glass transition temperature. Benzylation of cellulose fibers reduced E' of IR composite, while mercerization increased it. A tan 6 peak was observed at about -33OC at 110 Hz along with a shoulder peak at about -16ยฐC for the IR composites. The intensity of the tan 6 peak decreased when mixed with cellulose fillers. The shoulder peak next to the main peak could be separated assuming a Gaussian type equation; log
, where D is the maximum value of log E" vs. 1/T curve, To is the maximum temperature, and C is a parameter to determine the width of the Gaussian function. From the Arrhenius plot of the frequencies against l/T,,, the apparent activation energies were calculated as 220-235 kJ/mol for the shoulder peak (a1 process) and ca. 180 kJ/mol for the main peak ( a2 process). The a2 process is caused by the glass transition of the rubber matrix. The a1 process is assigned as the molecular motion of the rubber matrix strongly restricted by the fiber element.