Recent advances in experimental techniques for simultaneous measurements of size, concentration and temperature of multicomponent droplets [1] and the very diverse applications of nano- and microdroplets in physics, chemistry, biology, and engineering pose new demands to the theory of evaporation and condensation of multicomponent single droplets. From that point of view, our recent results [2-7] in the theory of a transient stage of
multicomponent droplet growth or evaporation, when concentration and temperature in the droplet gradually approach their stationary values before the steady rate of the droplet growth or evaporation establishes, should be of interest. The general equations of mass and heat transfer and balance between the droplet and vapor-gas environment have been analyzed. In
the case of steady mass and heat transport in the vapor-gas environment at substantial excess of the carrier gas, we have written integral relations for the droplet size, concentration and temperature and the relations determining the stationary values of concentration and temperature for non-ideal solution in the droplet. We have shown that there is a possibility of nonmonotonic change in the droplet radius in both isothermal and nonisothermal condensation or evaporation. Some numerical illustrations of time-dependent size,
temperature and composition of a condensing or evaporating microdroplet of water-ethanol and water-sulphuric acid solutions have been done in the diffusion regime of mass and heat transfer with account of non-ideality of the solutions. To include an arbitrary initial droplet size and component concentrations, two opposite initial concentrations of solution in the droplet
related to pure first and pure second components have been considered.