Effects of organically modified montmorillonites (OMMTs) with different type and amount of modifiers on flame retardancy of polystyrene (PS) have been studied. The results from morphology analysis, gas chromatography-mass spectrometry and cone calorimeter have showed different mechanisms for the flame retardancy of PS/OMMTs composites, depending on surface property of OMNTrs. One is the catalysis of acid sites formed on the surface of octadecylammonium modified MMT (c-MMT) via Hoffman decomposition on the carbonization of degradation products, which promotes the formation of clay-enriched char barrier. The other is related to chemical effect of inherent properties of MMT sheets on reactive intermediates during combustion, which work in PS/Na-MMT and PS/sodium dodecylsulfonate modified MMT (a-MMT) composites. Furthermore, the effect of the aggregate states of a-MMT on the flame retardancy of PS/a-MMT composites was studied. In the case of a-MMTs from spraying drying, a good protective layer with foam-like aggregate state of MMT sheet was formed in the residue, which results in low peak value of heat release rate of PS/a-MMT composites. (C) 2009 Elsevier Ltd. All rights reserved.