The solvothermal reduction of graphene oxide (GO), modified by the addition of vanadium chloride, resulted in an increased reduction degree of the reduced graphene oxide (rGO), which is reflected by a remarkably increased electrically conductivity of up to 8.5 S/cm, a value 30 times higher than that of rGO prepared without vanadium salt addition. Parallel with this increase, the thermoelectrical properties of rGO are improved, with a reached maximum Seebeck coefficient of 13.7 mu V/K. The rGOs were used as fillers in flexible styrene-butadiene-styrene triblock copolymer composites prepared by solution mixing. Compared to the traditionally prepared reduced graphene oxide, the new product provides up to 60 times higher conductivity to the composite, while the Seebeck coefficient is nearly the same. The highest power factor of 4.6 x 10(-4) mu W/(m.K-2) was achieved at 100 degrees C with 30 wt% loading, which is 30 times higher than that of the traditional reduced graphene oxide containing composite.