Poly(butylene/diethylene naphthalate) copolymers (PBN-PDEN) were synthesized in bulk according to the polycondensation procedure and examined by NMR, GPC, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and XRD. At room temperature they appeared as semicrystalline materials; the copolymerization caused a lowering in the T-g value, a decrement of T-m and of the crystallization rate. Pure alpha or beta' form was obtained at low and high DEN unit content, respectively; crystalline form transition never occurred in the solid state, analogously to PBN. After cooling from the melt, the pure alpha form was always evidenced in PBN-PDEN10 and PBN-PDEN20, whereas the pure beta' crystal phase develops in the copolymer containing 40 mol % DEN units, independently on the cooling rate. In the case of PBN-PDEN30 a pure alpha or beta' form was obtained at low and high cooling rate, respectively. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1356-1367, 2009