A theory is proposed for the rate of isothermal stationary nucleation of liquid or solid phases from vapors in the presence of inert carrier gas. Formulas are derived for the nucleation rate and the critical supersaturation as functions of the carrier-gas pressure. It is found that, depending on the interactions between the molecules of the carrier gas and the condensing vapors, the carrier-gas pressure can either stimulate or inhibit the nucleation process. The analysis shows that, typically, this effect is relatively small for carrier-gas pressures not much greater than the atmospheric pressure and is more pronounced for less supersaturated condensing vapors.