Local deformation of a polymer mixture crosslinked by irradiation with ultraviolet light was in situ monitored by using a Mach-Zehnder interferometer. In combination with the refractive index data obtained from independent measurements, the deformation in the nanometer scales of the crosslinked blends was calculated by using the difference in optical path length of the blend measured before and after irradiation. Upon varying the crosslink density of the blend by changing the light intensity, it was found that the local deformation well correlates with the crosslink density obtained from the reaction kinetics experiments. Furthermore, the strain relaxation of the blends was also monitored in situ and analyzed after irradiation over different time intervals. The results obtained in this study reveal the possibility of monitoring the nanometer-scale deformation in polymers during radiation curing. These data also provide important information on the correlations between the irradiation-induced elastic strain and the resulting morphology of reacting polymer blends. (c) 2005 Wiley Periodicals, Inc.