Abstract. Intrinsic fluorescence method was applied to study the miscibility and interactions of thermoplastic phenol formaldehyde resin (TPF) / poly(!-caprolactone) (PCL) blends. The characteristic intrinsic fluorescence emission of TPF at 313 nm showed the very good sensitivity to monitor the macromolecular chain motion in the TPF/PCL blends. The glass transition (T g ), crystallization (T c ), and melting transition point (T m ) of TPF/PCL blends were measured by the temperature dependence of intrinsic fluorescence intensities upon heating or cooling process. Interestingly, when TPF/PCL " 5/5, besides a T g for the amorphous phase of blend, another transition at temperature a little higher than T g of PCL can be observed by intrinsic fluorescence method. This microheterogeneity can be explained by the so-called 'rigid amorphous phase' (RAP) due to the good flexibility and the strong self-association of PCL chains in amorphous phase. Besides, the analysis of the dependence of T g on the content of PCL suggests that this microheterogeneity can attenuate the interactions between TPF and PCL chains and result in a lowering of T g s of blends. In view of the simplicity and sensitivity of measurement as well as affordability of instrument, intrinsic fluorescence proved to be an effective means for characterization of microstructural variation in polymer blends.