The miscibility of random fluorinated tetrafluoroethylene copolymers is investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). In particular, mixtures composed of poly(tetrafluoroethylene)-co-(hexafluoropylene) (FEP) containing 1 mol % of comonomer and poly(tetrafluoroethylene)-co-(perfluoromethylvinyl ether) (PFMVE) containing from 2 to 10 mol % of comonomer have been examined. The miscibility has been found to depend more upon the difference in the defect (counit) concentration between the two components than upon the blend composition and the crystallization conditions.