Layered mesostructured silicates and aluminosilicates with covalently attached hexadecyl groups (denoted as C-16-LMS, C-16-LMAS, and a sample with layers whose thickness was increased by additional silicate, C-16-SiO2-LMAS) were investigated as synthetic clays for dispersion and exfoliation in polymer melts. The dispersion of these clays in 13 organic solvents and their performance in polystyrene (PS) nanocomposites were examined. The three synthetic clays dispersed and formed gets in aromatic solvents and in a branched alkyl solvent (2,6,10,14-tetramethylpentadecane, TMPD) based on visual observations and rheology. The elastic moduli (G') of the toluene/clay dispersions for all three clays were similar when compared at equal inorganic content. The synthetic clays were blended with PS samples of various molecular weights. Melt rheology of the PS/clay nanocomposites showed a dramatic increase in elastic modulus compared with neat PS and formation of a G' plateau at low frequencies. The plateau occurred at higher G' values for C-16-LMAS than for C-16-SiO2-LMAS or C-16-LMS, indicating that C-16-LMAS has higher strength and/or higher aspect ratio and can thus withstand the stresses of melt mixing. Increasing the molecular weight of PS increased G' of the PS/C-16-LMAS nanocomposites. By small angle X-ray (SAXS) and transmission electron microscopy C-16-LMAS showed better dispersion and a higher aspect ratio in the PS-nanocomposite than C-16-SiO2-LMAS. © 2005 Elsevier Ltd. All rights reserved.