An adaptation of an extrinsic Fabry-Perot interferometer (EFPI) strain sensor is described, which permits the state of cure of an epoxy matrix to be monitored, when the sensor is embedded in a polymeric matrix composite. By using a glass rod with a retroreflecting end for the target fiber in the EFPI sensor, the intensity of the light reflected depends on the refractive index of the host matrix, if a low coherence source is used. As the epoxy cross-links during cure, the refractive index of the epoxy will increase to a value exceeding that of the target fiber. The resulting increased loss in the fiber can be detected at the sensor output and correlated to the state of cure of the epoxy. After cure, the sensor may be operated as a conventional extrinsic Fabry-Perot interferometric strain sensor if a coherent source is used. Using the modified extrinsic Fabry-Perot sensor, we monitor the cure of Devcon® 5-Minute® Epoxy, and show that it cures in approximately 60 minutes.