A new type of filler, composed of only ultrafine particles (silica fumes), has been used to design mastics and asphalt concretes. An experimental campaign on mastics and mixtures, performed at the ENTPE/DGCB laboratory, compares the effect of the ultrafine particles to that of "classical" fillers. The linear viscoelastic properties (shear complex modulus G*) of mastics have been measured with a specifically developed device (annular shear rheometer) presented in this paper. The potential for reinforcement of fillers is quantified by the complex reinforcement coefficient R* sub M introduced in this paper. The results show that the use of the ultrafine particles greatly increases the complex modulus of mastics at high temperature, in comparison to mastics made with classical fillers. In the low-temperature region, the complex modulus is little affected by the filler characteristics. The effect of ultrafine particles has also been analyzed for asphalt concretes, which have been tested using a tension compression test on cylindrical specimens, in the small-strain domain. The analysis reveals that the complex modulus E* is higher for the materials containing ultrafine particles at high temperature, as observed for mastics.