Poly(vinylidene chloride-co-vinylchloride)/organically modified hectorite (VDC-VC/SPN) nanocomposites were prepared by melt blending VDC-VC copolymer with SPN in the presence of dioctyl phthalate, which acted as a plasticizer. As a result, the exfoliated structure was found in the VDC-VC/SPN nanocomposites. In nitrogen atmosphere, VDC-VC/SPN nanocomposites exhibited a single-step thermal degradation. The thermal stability of VDC-VC/SPN nanocomposites is significantly influenced by the SPN, which was modified with long alkyl ternary ammonium salt. In air atmosphere, VDC-VC/SPN nanocomposites revealed a two-step thermo-oxidative degradation behavior. At the first degradation stage, the weight loss pattern is similar to that of VDC-VC composites in nitrogen, in which the thermo-oxidative stability of VDC-VC/SPN nanocomposites is affected by the ternary ammonium salt and oxygen rather than its morphology. At the second degradation stage, both the enhanced thermo-oxidative stability and the flame-retardation ability of VDC-VC composites are strongly and closely related to the morphology of nanocomposites. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3171-3180, 2009