The compatibilization effects provided by polymethacrylic ionomer (PMMA ionomer) on monomercasting polyamide6 (MCPA6)/clay (pristine sodium montmorillonite) composites were studied in this work. The PMMA ionomer used in this study was sodium polymethacrylate ionomer (PMMA Na+-ionomer), which is a copolymer of methyl methacrylate and sodium methacrylate, prepared using emulsion polymerization. MCPA6/clay/PMMA Na+-ionomer composites were prepared by in situ anionic ring-opening polymerization (AROP) of epsilon-caprolactam (CLA). X-ray diffraction (XRD) and transmission electron microscopy (TEM) plus rheological measurement were used to characterize those composites. The results indicated that PMMA Na+-ionomer is a good compatibilizer for this system. With increasing PMMA Na+-ionomer content, a better dispersion of clay layers was successfully achieved in the MCPA6 matrix. Furthermore, differential scanning calorimetry (DSC) and XRD results indicated that well dispersed silicate layers limit the mobility of the MCPA6 molecule chains to crystallize, reduce the degree crystalline, and favor the formation of the gamma-crystalline form of the MCPA6 matrix. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 2727-2732, 2008