The ionic mobility, μ_i, of a species, i, is the velocity, v of a particle that moves under the influence of an electric field, E, of unit strength:

The dimension of the mobility is square meter per volt per second. The values of the mobilities range from more than 300 units for the proton in water to about 5 units for large organic ions in solvents with high viscosities. The mobility depends on the size and shape of the solvated ion, on its charge, and on the viscosity and temperature of the solution. Thus, it is clear that the mobility is a function of the solvent.

Three kinds of mobility can be distinguished: the absolute mobility, μ⁰ in an infinitely diluted solution, the actual mobility of the fully charged ion at the ionic strength, I, of the solution, and the effective mobility, μ^eff, which depends on the degree of ionization, .