Bio-based copolyesters containing sulfonate functions (PEFSI) were synthesized by a melt step-growth polymerization of furanic and sulfonated isophthalic diesters with ethylene glycol. The monitoring of the polymerization by H-1 NMR allowed the detection of etherification side reactions and highlighted the blocky character of copolyesters. Thus optimizing catalysts (Zn(OAc)(2) and Ti(OBu)(4)) and adding inhibitor (NaOAc) were found helpful to optimize the process leading to copolymers of controlled dioxyethylene units (7-9 mol%) and number average molecular weights higher than 19 000 g mol(-1). The thermal properties (DSC, TGA) highlighted the role of sulfonated isophthalate units which both rigidify and stabilize the copolyester backbone giving rise to higher Tg and thermal stability than non sulfonated homopolymer (PEF). The water sorption mechanism was also discussed as a function of the copolymer composition and the prevailing role of the sulfonated group content on the water uptake was evidenced. Finally the hydrolytic degradation has again confirmed the role of the sulfonated units and underlined those of the blockiness character of the macromolecules. (C) 2013 Elsevier Ltd. All rights reserved.