TEM studies have been performed in order to evaluate the amplitude and the uniformity of the strain field of epitaxial layers. Using adequate two-beam conditions, moire patterns allow us to measure independently the misfit strain components parallel and normal to the interface. The GaSb/(001)GaAs system has been studied by this technique. The measured spacings of moire fringes correspond well with the values calculated with the bulk parameters, thus indicating a complete relaxation. The mismatch accommodation is obtained by the direct creation of a Lomer dislocation network. In Ga0.8In0.2As/(001)GaAs system, the 10 nm thick epilayers do not give any contrast related to either organised misfit dislocations or strained layers. These observations show that the real strained state of the epitaxial layer is different from that predicted by the elastic theory. In fact the misfit stress, induced by a few percent of In, is high enough to perturb the ideal 2D growth: an indium segregation creates a transient composition with a random distribution of group III elements. This chemical behaviour induces a transition zone, between the substrate and the uniform epilayer, which contributes to the strain relaxation. (C) 2002 Elsevier Science B.V. All rights reserved.