The photo-fluorescence of a solution-cast film containing a 30/70 blend of poly(p-phenylene benzobisoxazole) (PBO)/amorphous nylon shows a substantial decrease in fluorescence yield, and a blue-shifted spectrum upon annealing at around 220-degrees-C, corresponding to the transition from an entrapped single phase to a multiphase system. This indicates a substantial dependence of PBO fluorescence on its domain structure in this blend.