In-line studies of the initial stages of shear-induced coalescence in two-phase polymer blends were carried out with a home-built device combining a cone and plate rheometer and a fiber-optic-assisted fluorescence detection system. A blend of 90 wt % poly(2-ethylhexyl methacrylate) (PEHMA) and 10 wt % poly(butyl methacrylate) (PBMA) was prepared by the casting of films onto a solid substrate from mixed aqueous latex dispersions of the two polymers. The dispersions were prepared via emulsion polymerization under conditions in which both components were formed as spherical particles with a very narrow size distribution. By using a 14:1 particle ratio of PEHMA to PBMA, we obtained films in which 120-nm PBMA particles were surrounded by a PEHMA matrix. The blend contained phenanthrene-labeled PBMA particles and anthracene-labeled PBMA particles in a ratio of 4:1, whereas the PEHMA matrix polymer was unlabeled. We monitored the anthracene-to-phenanthrene fluorescence intensity ratio (I-470/I-360) as a measure of direct nonradiative energy transfer from phenanthrene to anthracene, whereas the blend was sheared at different shear rates and temperatures. Under no-shear conditions, the results of in-line experiments were in good agreement with the results of off-line measurements of energy transfer by conventional techniques. In blends under shear, the two sets of experiments, in-line and off-line, did not agree with each other. The cause of this disagreement was associated with normal forces in the blend under shear that affected the optical path length and the relative intensities of the fluorescence signals of the phenanthrene and anthracene groups in the blend.