The structural and residual stress in carbon-fiber reinforced polymeric materials can be evaluated by measuring the amount of strain in embedded particles which are sensitive to X-ray diffraction. This was first done by BARRETT & PREDECKI [1,2], subsequently by CHAUHAN & HAUK [4] and WORTLER, PRINZ & SCHNACK [10,12]. An overview on the subject was recently given by HAUK [4]. The main advantadge of the method is that the full three-dimensional stress tenser in the interior of the composite (depending on the location of the filler particles) can be evaluated from the measured strain in the particles. This gives the possibility to measure interlaminar stress concentrations in multidirectional laminates such as the free-edge stress state? which is the main factor for the onset of delaminations in the material. Therefore a model is proposed for the effective stress-strain relationship between the strain in the particles and the stress in the surrounding composite. The model is derived from a micromechanical approach and the consistency of the model is checked by Finite-Element computations of representative volume elements. The predictions of the model for the effective stress-strain relationship are compared to experimental results and conclusions about the validity of the model are drawn.