The effect of the incorporation of small concentrations of reduced graphene oxide (< 1 weight %) on the viscous and elastic properties of HDPE and PP are analyzed. The addition of Poly-Ethylene Oxide (PEO) as compatibilizing agent is also explored. The results of the dynamic viscosity do not reflect any effect of the rGO and/or the PEO in the polyolefin matrices, because the obtained curves are very similar in all cases. The measurements of the elastic modulus of composites, without compatibilizer, are also reported. These measurements indicate that the rGO do not act as reinforcement, but as impurities, since the magnitude of the composite modules is less than those of the neat polyolefin. This situation is very different when PEO is added. Using percentages of rGO as low as 0.1% and 2% of PEO in composite, obtained values of elastic modulus for composite are almost four times than that of neat HDPE and nearly three times than modulus of neat PP. Increasing the percentage of rGO to 0.5%, with the same PEO percentage, the elastic moduli of composites are around three times the value of the corresponding modulus for the two neat polyolefins.