The consistent and computationally efficient stochastic statistical approach is suggestedto study kinetics of decomposition of metastable alloys. An important parameter of the theory is thesize of locally equilibrated regions at the nucleation stage which is estimated using the ``maximumthermodynamic gain'' principle suggested. For several realistic models of Fe-Cu alloys studied, the re-sults obtained agree well with kinetic Monte Carlo simulations. Application of methods developed tostudies of decomposition of Fe-Cu-Mn alloys revealed a great sensitivity of evolution to both the con-figurational and kinetic interatomic interactions in an alloy. Using for these interactions the availablefirst-principle and CALPHAD estimates, we can well reproduce the peculiar features of decomposi-tion of Fe-Cu-Mn alloys observed in experiments.