This paper is concerned with the structures and properties for fluorene-based polyester-polycarbonate (FBP/PC) alloys. Obtained alloys were characterized thermodynamically, optically, and viscoelastically, and the relationship between drawing behavior and molecular dynamics were also investigated. FBP/PC alloys showed transparent and a single glass transition temperature (Tg) for all compositions that indicated the complete compatibility in FBP/PC alloy systems. Tendency of the maximum draw ratios for alloy sheets was very similar to the profile of Tg versus FBP content. Maximum draw ratio was increased linearly with FBP content, demonstrating the intimate relationship between the large deformation and the molecular motion. A large amount of orientational birefringence occurred in PC sheet, but in the case of FBP/PC alloys and FBP, orientational birefringence was drastically decreased or not observed at all. This meant that positive birefringence of PC molecules was compensated by the absolutely smaller birefringence of FBP molecules. From a molecular mobility perspective, relaxation time (T2) for PC was smaller even at 200 degrees C. However, for FBP and its alloys, T2 were much larger, showing the considerably enhanced molecular mobility in FBP or FBP/PC alloys. In this study, we could first reveal that the relationship between Tg and FBP content was very similar to the behaviors of birefringence (Delta n)/draw ratio (lambda) versus FBP content, T2 versus FBP content and 1/maximum draw ratio (lambda(max)) versus FBP content. Based on these results, we were able to propose a novel alloy with high refractive index, low orientational birefringence, and higher processability by alloying FBP with PC. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 461-468, 2009