A new method for measuring the ion mobility of precritical clusters in supersaturated vapors is reported. The method is based on the REMPI (Resonance Enhanced Multiphoton Tonization) nucleation technique, which allows the generation of selected ions within a supersaturated host vapor held in a diffusion cloud chamber. The precritical cluster ions drift under the influence of a uniform electric field to the nucleation zone, where they become condensation nuclei and rapidly grow into macroscopic liquid droplets that can be detected by light scattering. The reduced mobility of the precritical clusters in the toluene(+)/methanol and styrene(+)/methanol systems decreases with increasing methanol supersaturation, which reflects the increase in the size of the solvated ion in agreement with Thomson's model for ion-induced nucleation.