The relaxation of InGaAs/GaAs thin films grown by molecular beam epitaxy is studied by an in situ optical stress sensor technique. Profiles of normalized film strain versus thickness are obtained for several growth temperatures and two InGaAs compositions. The profiles are modeled using Dodson and Tsao's concept of effective stress, and a new kinetic model of dislocation sources associated with the surface roughness, which could be generated by film growth and have a transient response following a growth interrupt. (C) 2002 Elsevier Science B.V. All rights reserved.