Composites of polystyrene (PS) and poly (styrene-co-acrylonitrile) (SAN) containing a fraction of a dodecylsulfate-modified Mg-Al layered double hydroxide (LDH) were prepared by means of a melt-extrusion process. The structure and morhology were analyzed with wide-angle X-ray scattering and transmission electron microscopy, respectively. The X-ray spectra of the PS matrix composite displayed the diffraction peak characteristic of the hybrid LDH basal plane at 20 = 3.1 deg. The SAN matrix composite did not exhibit such a diffraction peak. Both PS and SAN composites displayed an intercalated type of morphology with respect to the LDH platelets, as assessed by transmission electron microscopy. A plasticizing effect due to the hybrid LDH particles was observed for all composites and was supported by a decrease in the glass-transition temperature values and by Fourier transform infrared spectra. Besides tensile properties, the fracture toughness of the composites was compared with that of the pure polymers through the linear elastic fracture mechanics parameters. They were determined from fracture tests under a three-point-bending configuration. The results indicated that the effect of adding a small fraction of modified LDH particles to SAN caused an improvement in fracture toughness of 50% with respect to that of the pure polymer. Moreover, the relative increase in the fracture energy was about 200%. For PS matrix composites, both tensile properties and linear elastic fracture mechanics fracture parameters remained unaffected. These results were explained on the basis of the different plasticities developed by both polymers around the particles. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2574-2583, 2009