Isothermal condensation of supersaturated vapors of various substances and formation of lampblack particles by isothermal decomposition of hydrocarbon (acetylene) molecules is investigated by analytic methods. Expressions that differ generally from the classical ones are obtained, on the basis of the quasichemical model of cluster particles, for the quasi-equilibrium and stationary distribution functions of the cluster sizes, as well as for the rates of formation of clusters having critical dimensions. Analytic expressions describing the time evolution of the degree of supersaturation and of the size distribution function of condensed cluster particles are obtained for the case when the system goes over from the supersaturated state to thermodynamic equilibrium. Equations for the principal characteristics of lampblack formation, viz., the characteristic time and the average size and density of the lampblack particles, are obtained with allowance for the properties of the formation and dissociation (evaporation) of small lampblack cluster particles.