The formation of aerosol particles on molecular nuclei of condensation (MoNC) is considered. An interpretation of this phenomenon, consistent with the physicochemical features of MoNC and with the observed regularities, is proposed. The nucleation is induced by the addition of two or several vapor molecules of a developing agent to MoNC to give clusters that are more stable than spontaneous heterogeneous fluctuations. The physical properties of the supersaturated vapors of empirically selected developing agents conform to the condition of low concentrations of heterogeneous fluctuations, which is needed to observe MoNC.