The sub-T-m exotherms in polyamide 6 (PA6) have been carefully re-examined by differential scanning calorimetry and X-ray diffraction, considering the effects of processing and thermal history, addition of water and clay. The results obtained cast doubt on Khanna's proposal that sub-T-m exotherm, in PA6 comes from the release of strain energy absorbed during processing, and suggested that the origin of sub-T-m exotherm is the gamma-alpha transformation at the premelting temperature, namely, the less thermodynamically stable gamma-form (gamma(ns)) transforming into the more thermodynamically stable alpha-form (alpha(s)). The presence of water or clay in PA6 samples facilitated the formation of gamma(ns) at corresponding cooling rates, and enhanced the development of sub-Tm exotherms. During the heating scan of PA6/clay composites, the initial gamma(ns) can be transformed into more stable (gamma(s))(t) and alpha(s) at the same time, which can be thought as the origin of their sub-Ton events. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2385-2393, 2009