X-rays can be used to measure residual as well as applied stresses in polymers and reinforced polymeric composites, by diffracting from filler particles that are embedded before curing. We have investigated various fillers that exhibit suitable stress induced shifts of diffraction angle when embedded in uniaxial graphite fiber/epoxy composites. X-ray measurements of elastic strains in the particles are proportional to the corresponding composite strains, in agreement with the model of H. T. Hahn. Results indicate that the stress sensitivity (change in X-ray diffraction angle per MPa applied to the composite) increases in the order W, CdO, Hi, Ag, Nb, Al. With Al 333 + 511 reflections of CuKα1, the diffraction angle shifts 8.52 x 10-4 deg 2θ per MPa. Theoretical predictions are in reasonable agreement with this figure. The elastic range terminates at a yield point beyond which little or no further shift of the diffracted beams is seen for metallic fillers, but no such yield point has appeared with CdO particles. Oppositely directed shifts are seen after unloading from stress levels that have exceeded the yield point.