Epoxy resin/montmorillonite nanocomposites were obtained via in-situ intercalative polymerization. Na-montmorillonites modified with octaclecyl, hexadecyl and dodecyl trimethylammonium, benzyl triethylammonium and tetramethylammonium ions were used in diglycidyl ether of bisphenol-A epoxy resin. Montmorillonites modified with long alkyl ammonium ions yielded intercalated nanocomposites whereas those with benzyl or very short alkyl ammonium ions gave phase separated microcomposites. Significant improvements in flexural strength and stiffness were obtained, especially at a low clay mineral level of 0.5 mass% with octaclecyl trimethylammonium ions. Fracture toughness values double that of neat epoxy resin could be reached together with improved stiffness and strength by nanocomposite formation. The largest improvement in fracture toughness was also obtained by octadecyl trimethylammonium montmorillonite. Regarding flexural strength and fracture toughness, optimum clay mineral levels were assessed as 0.5 mass% for intercalated nanocomposites. Flammability was evaluated by oxygen index measurements with varying clay mineral level and surface modification. Largest improvement, i.e. from 20.9% to 23.2%, was attained with 2 mass% dodecyl trimethylammonium montmorillonite containing epoxy resin. Basal spacings in char residues of oxygen index tests were around 1.3 nm, irrespective of the montmorillonite content, type of modification and oxygen index performance. (C) 2009 Elsevier B.V. All rights reserved.