Condensation of a supersaturated vapor of n-butanol on monodisperse submicrometer particles of lactose and monosodium glutamate is investigated in a how cloud chamber (FCC). The dependence of critical supersaturation S-cr on the particle size in the range 30 to 90 Nn is experimentally examined. The results show that the size dependence of S-cr qualitatively agrees with that predicted by the Fletcher version of the Volmer theory of heterogeneous nucleation, but to a lesser degree. The experimental S-cr is smaller than the theoretical prediction even with the line tension and surface diffusion taken into account, and they induce heterogeneous nucleation better than perfectly wetted particles. The discrepancy can not be fully accounted for by the effects of line tension and surface diffusion and the existing theory concerning the curvature-dependent surface tension. The condensation on single positive-charged particles of diameter 30, 60, and 90 nm is also examined. A lowering of S-cr at an efficiency much larger than the prediction by Volmer's theory for ion-induced nucleation is observed, and the charge effect fades away as particle size increases.