We investigate the phase transformation in non-stoichiometric GaAs films, which are doped with group V isovalent impurities, namely Sb and P. In contrast to Al and In atoms of group III, group V atoms may not only form substitutional alloys with the GaAs matrix, but also can be dissolved in As precipitates. Our experimental study based on transmission electron microscopy, x-ray diffraction and optical characterizations revealed an opposite impact of the P and Sb alloying on the nucleation, growth and coarsening of the second phase. While Sb enhances the precipitation rate, P retards it. Delta-doping with Sb causes two-dimensional precipitation, whereas such doping with P does not result in heterogeneous nucleation. The microstructure and strains have been found to be different around nanoinclusions in the Sb-doped and P-doped materials, indicating different thermodynamics and kinetics of segregation of the two isovalent impurities in As nanoclusters. We analyze the observed phenomena in terms of thermodynamic and kinetic models taking into account the underlying phase equilibriums and diffusion mechanisms.