Using previously published relaxation models [D. J. Dunstan, P. Kidd, L. K. Howard and R. H. Dixon, Appl. Phys. Lett.59, 3390 (1991) and D. Gonza´lez, D. Arau´jo, G. Arago´n, and R. Garcı´a, Appl. Phys. Lett.71, 2475 (1997)] that predict the strain relaxation in the InGaAs/GaAs system, before and during the stage of relaxation saturation, the critical thickness where dislocation interactions begin to limit the plastic relaxation is estimated. The approximations used to deduce an analytical expression are shown to be appropriate for describing the regime of relaxation considered. A good agreement with experimental data previously published by other authors permits a physical explanation for the different observed regimes of relaxation to be given. ©1998 American Institute of Physics.