The thermal behavior of blends of nylon 6,6, with an amorphous polyamide, Trogamid-T, and a semicrystalline polyamide, nylon 6,12, was studied. The blends were prepared both by solution blending and by melt blending, using a Maxwell extruder and a twin screw extruder. The concentration of the blends ranged from 75% to 95% by weight of nylon 6,6. Annealing the blend samples in the molten state in a differential scanning calorimeter (DSC) produced changes in the melting and crystallization behavior. This was attributed to transamidation reactions occurring between the blend components, leading to the formation of in-situ block copolymers. The length of the blocks decreased with annealing time, as suggested by reduced melting (T-m) and crystallization temperatures (T-c) and heat of fusion values. The changes in thermal behavior were dependent on the blending method, additive concentration, presence or absence of a catalyst, melt annealing time, and the extent of melt mixing. The extent of reaction, measured by the depression in equilibrium melting temperature, was linear with respect to the annealing time. The Trogamid-T containing blends appeared to be "nearly miscible" while those with nylon 6,12 were initially immiscible. The glass transition temperature (T-g) vs. the composition curve of the nylon 6,6,6/Trogamid-T blends showed a positive deviation from linear additivity, with the single T-g decreasing as a function of annealing time in the melt.