Summary: Diglicidylether of bisphenol A (DGEBA) was blended with poly(L-lactic) acid (PLLA) and cured with diaminodiphenylmethane (DDM). The uncured blends were miscible but became immiscible upon curing. The relationship between phase morphology and thermal stability was investigated. It was found that the thermal degradation of uncured DGEBA, PLLA and their blends occurred mainly through one step. PLLA and uncured DGEBA were mutually destabilized. This is probably due to chemical effects, such as some sort of interaction between the products of degradation of DGEBA and PLLA in the miscible blend. On the other hand, the biphasic cured DGEBA/PLLA blends displayed two degradation stages associated to decomposition of two phases, one rich in PLLA and the other in DGEBA. The temperature at which weight loss begins in the blend is intermediate to the temperature of the neat components. This was attributable to physical effects.