Polyurethane elastomers based on castor oil or polybutadiene liquid hidroxyl (PBLH), as well as polyurethane composites reinforced with coir or sisal fibers at different proportions: 5, 10 and 20% w/w were prepared. The thermal behavior of the samples was analyzed by thermogravimetry (TG/DTG). TG/DTG curves showed two decomposition stages, except samples from castor oil with three decomposition stages (Tonset 257 degrees C). The PU from PBLH presented a principal degradation at around 400 degrees C, but with 15% weight loss at 250 degrees C and the composites exhibited first decomposition stage at around 290 degrees C. MDSC thermal profiles for PBLH PU presented one transition, occurring at the glass transition temperature (T-g) 73.8 degrees C, two transitions in castor oil PU T-g: -1.6 and 62.7 degrees C, and three transitions for composites from sisal T-g: -52.9 degrees C, 66.7 and 122 degrees C. These transitions suggest soft and hard segments.