The microphase structure in blends of poly(n-butyl methacrylate-co-styrene) (nBMAS) and polystyrene (PS) was investigated by nonradiative energy transfer (NRET) using both steady-state fluorescence and time correlated single-photon counting (TCSPC) measurements. The nBMAS was labeled with anthryl groups by semicontinuous emulsion copolymerization. Studies of the morphology and phase separation of these blends performed by epifluorescence microscopy were complemented by differential scanning calorimetry. Miscibility of polymers with several compositions, from 5 to 95% w/w, was studied. For compositions with PS < 80 wt % we obtained a partial miscibility with discrete domains of PS dispersed in a nBMAS matrix. Blends up to PS = 20 wt % presented a co-continuous morphology whereas a bicontinuous morphology was generated for lower PS content. Fluorescence lifetimes and the lifetime distribution were consistently associated with the microheterogeneity of the material and were considered an additional criterion for the analysis of miscibility.1081339753984