Environmental considerations have been the cause of increasing research and development of waterborne polymer systems for many different applications, particularly as coatings for several kinds of substrates. In this work, waterborne polyurethanes (WPU) based on block copolymers of ethylene glycol and propylene glycol (EG-b-PG), with 25% of EG segments, poly(propylene glycol), dimethylolpropionic acid, isophorone diisocyanate, and hydrazine, as chain extender, were obtained in the absence of organic solvent. Thermal stability, by thermogravimetry, and mechanical properties of cast films obtained from the aqueous dispersions were evaluated. The degradation process started above 200 A degrees C for all samples. It was verified that the block copolymer improved the thermal resistance but diminished the mechanical resistance of the WPU materials.