The thermal stability of several crystal polymorphs in polyamide 11 (PA11) and polyamide 6 (PA6) has been investigated by means of in situ X-ray experiments. In the case of PAH, delta' and alpha' phases display a Brill transition far below the melting point. Both phases transform into a (pseudo)-hexagonal HT delta phase above 100 degrees C. The latter turns back into the most stable alpha' phase upon cooling. PA6 exhibits similar thermally induced crystal transitions. In situ X-ray investigations refute the occurrence of a beta -> alpha transition upon heating, whereas the existence of a pseudohexagonal HT phase is suggested as in PA11. For both polymers, the present study underlines the major role of crystal perfection of the most stable alpha phase on the existence of a Brill transition. The combination of in situ structural information with thermal analysis allows to propose a thermodynamic scheme to describe the Brill transition in both polyamides.