Surface and interface stress evolution is measured in situ and in real time during molecular beam epitaxy of InAs/GaAs and GaSb/GaAs systems, covering initial 2D growth, formation of QDs and subsequent GaAs capping. These two systems with very similar lattice parameter mismatch but involving quire different surface chemistry show a similar critical thickness for 2D/3D transition, 0.7-1.0 monolayer ML. Limited In incorporation during InAs/GaAs growth affects the relaxation process. In segregation effects are observed during GaAs overgrowth. GaSb/GaAs system presents a strong 2D/3D relaxation associated with mass transport from the 2D layer. Sb/As exchange reactions are considered.