Randomly branched poly(butylene isophthalate) samples containing sodium sulfonate groups were prepared from dimethyl isophthalate, 3,5-bis(carbomethoxycarbonyl) benzene sulfonate, tris(hydroxyethyl) isocyanurate, and 1,4-butanediol, according to the well-known two-stage polycondensation procedure. The polymers, containing various amounts of branching units and ionic groups, demonstrated to be soluble in the most common organic solvents, an evidence that gelation was not reached under the polymerization conditions adopted. The thermal behavior was examined by thermogravimetric analysis and differential scanning calorimetry. The sulfonate as well as the branching units had only a limited effect on the thermal stability, which slightly decreased with respect to pure poly(butylene isophthalate). The analysis carried out using DSC technique showed that the T-m of the copolymers decreased with increasing counit content, differently from T-g, which, on the contrary, increased. Baur's equation was found to describe well the T-m-composition data. (c) 2005 Wiley Periodicals, Inc.