Nucleation of intermetallic compounds and of voids at interfaces during reactive diffusion is treated with account of influence of the flux divergence in the nucleation regions of the real space as an additional term for drift in the size space (in Fokker-Planck equation for nucleation). Such approach enables the construction of effective Gibbs nucleation barrier which may (in the broad region of parameters) increase to infinity meaning the full suppression of nucleation, or, by the contrast, decrease assisting the nucleation. The introduced effective nucleation barriers depend on kinetic factors - on the ratio of diffusivities in nucleating and in neighboring phases. Thus, the competition of stable and metastable phases is reconsidered, as well as nucleation of Kirkendall/Frenkel voids at the interfaces.