We report on calculations of the reduced sedimentation velocity U/U-0 in homogenous suspensions of strongly and weakly charged colloidal spheres as a function of particle volume fraction phi. For dilute suspensions of strongly charged spheres at low salinity, U/U-0 is well represented by the parametric form 1 -p phi(alpha) with a fractional exponent alpha = 1/3 and a parameter p similar or equal to 1.8, which is essentially independent from the macroion charge Z. This nonlinear volume fraction dependence can be quantitatively understood in terms of a model of effective hard spheres with phi-dependent diameter. For weakly charged spheres in a deionized solvent, we show that the exponent alpha can be equal to 1/2, if an expression for U/U-0 given by Petsev and Denkov (1992, J. Colloid Interface Sci. 149, 329) is employed. We further show that the range of validity of this expression is limited to very small values of phi and Z, which are probably not accessible in sedimentation experiments. The presented results might also hold for other systems such as spherical proteins or ionic micelles.