Poly(butylene terephthalate-co-thiodiethylene terephthalate) copolymers of various compositions were synthesized and characterized in terms of chemical structure and molecular weight. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. All the polymers under investigation show a good thermal stability. At room temperature they appear as semicrystalline materials: the main effect of copolymerization was a lowering in the amount of crystallinity and a decrease of melting temperature with respect to homopolymers. A pure crystalline phase has been evidenced at high content of butylene terephthalate or thiodiethylene terephthalate units and Baur's equation was found to describe well the T-m-composition data. Amorphous samples (containing 50-100 mol% of thiodiethylene terephthalate units) showed a monotonic decrease of T-g as the content of sulfur-containing units is increased, due to the presence of flexible C-S-C bonds in the polymeric chain. Finally, the Fox equation described well the T-g-composition data.