The sedimentation velocity of dilute suspensions of spheres, with particle volume concentration up to 5%, has been experimentally measured for a variety of solid-liquid systems covering a large range of Reynolds numbers (from 0.01 to 1000). For the systems experimentally investigated, it was found that the settling velocity was a linear function of the particle volume concentration, the dependency being less marked as the Reynolds number increased. A comparison between dilute and concentrated systems behaviour suggested a possible improvement of the Richardson-Zaki equation.