A sub-melting-temperature exotherm in a polyamide-6/clay nanocomposite (containing 3 wt % montmorillonite) was investigated with differential scanning calorimetry. It existed only via air-quenching from the melt; it did not exist at higher or lower heating rates. The exotherm could be ascribed to frozen-in stresses in the interlamellar regions through hydrogen bonding. A combination of larger internal stresses and larger crystallinity was necessary to produce this exotherm. Its appearance was closely connected to the addition of montmorillonite. During the air-quenching process, montmorillonite not only greatly accelerated the crystallization rate of polyamide-6 but also further intensified the internal stresses produced during the quenching process. (C) 2005 Wiley Periodicals, Inc.