Composites based on thermosetting resin and reinforcement fibers present generally unwanted residual internal stresses inherent to their elaboration process. In this article, different curing experiments of thermosets (isothermal and anisothermal) were monitored using optical fiber Bragg grating (FBG) sensors and thermocouples, to assess the cure-induced strains. A thermokinetic model evaluates the degree of conversion of the resin. At the onset of stress transfer to the optical fiber, the degree of conversion ranges between 0.63 and 0.68. During curing, the FBG deforms under chemical shrinkage with an amplitude widely lower than the expected chemical strain, showing that the FBG signal is not directly related to the actual chemical shrinkage. However, once the resin is cured, the FBG sensor provides directly the coefficient of thermal expansion of the resin, as function of temperature and degree of conversion, reached in the different experiments. (C) 2013 Society of Plastics Engineers