The thermal behavior of linear and randomly branched poly(butylene isophthalate) samples was investigated by thermogravimetric analysis and differential scanning calorimetry. As to the thermal stability, it was found to be good and similar for all the samples. The thermal analysis carried out using DSC technique showed that the melting temperature of the polymers decreased with increasing branching unit content, although the glass-transition temperature was practically not affected by ramifications. The multiple endotherms typical of linear PBI were also observed in branched samples and were found to be influenced both by temperature and degree of branching. By applying the Hoffman-Weeks' method, the equilibrium melting temperatures of the polymers were obtained. The presence of a crystal-amorphous interphase was evidenced only for the branched samples and the interphase amount was found to increase as the branching unit content was increased. Isothermal melt crystallization kinetics was analyzed according to Avrami's treatment. The introduction of branching points was found to decrease the overall crystallization rate of poly(butylene isophthalate). Values of Avrami's exponent n close to 3 were obtained for all the samples, in agreement with a crystallization process originating from predetermined nuclei and characterized by three dimensional spherulitic growth.