The thermal degradation of some blends of polyamide 6/polycarbonate (PA6/PC) and polyamide 6/polycarbonate/poly(propylene oxide) (PA6/PC/PPO) were investigated. The copolymer formed during the mixing of polyamide 6 and polycarbonate, at 240 degreesC, for 30 min, increases the thermal stability of PA6/PC and of PA6/PC/PPO blends. This increase in the thermal stability occurs due to the plasticizing effect of PPO, which increases the mobility of the molecules of PAG and PC, and consequently increases the probability of the reaction between the -NH2 and -O-CO-O groups of polyamide 6 and polycarbonate, respectively. The ternary blends with PPO (5-10% w/w) have lower thermal stability than PA6/PC blends. This is due to the decrease of miscibility between these polymers and the rise of the diluting effect.