The present work deals with the synthesis of poly(methyl methacrylate-co-styrene)/montmorillonite [P(MMA-co-St)/O-MMT] nanocomposite using ultrasound assisted emulsion copolymerization operated in a semibatch manner. The synthesis process is based on dispersing montmorillonite (MMT) clay in the monomer (styrene) and surfactant (sodium dodecyl sulfate) under the influence of ultrasonic irradiations. Investigations have been carried out using treated MMT clay by quaternary ammonium salt (octadecylamine) as a starting material for establishing the dependency on the stability of emulsion and formation of latex. X-ray diffractrogram (XRD) have clearly established the complete exfoliation of MMT clay into the polymer. The exfoliated structure of nanocomposites has also been confirmed by transmission electron microscopy (TEM). It has been observed that both polymerization rate (R-P) and the fractional conversion decreased with an increase in the O-MMT clay loading in the emulsion polymerization system. The zeta potential and particle size analysis showed that nanocomposite latexes were electrostatically stable and average particle size was in the range of 156.58 to 191.23 nm with narrow particle size distribution. It has been observed that the exfoliated P(MMA-co-St)/O-MMT nanocomposite exhibits a higher glass transition temperature (T-g = 152.7 degrees C) and lower heat of reaction (Delta H = -265 J/g) at 1% O-MMT loading as compared to the neat copolymer (T-g = 127.3 degrees C, Delta H = -437.5 J/g). Nanocomposite formed using current method have been shown to give better thermal stability attributed to the interaction of O-MMT platelets with polymer leading to cross-linking enhancement. (C) 2011 Elsevier B.V. All rights reserved.