The effect of the presence of organomodified nano-montmorillonite (MMT) on the free radical polymerization kinetics of either ethyl methacrylate or butyl methacrylate was investigated. The in situ polymerization technique was selected with dispersion of the MMT nanoparticles into the corresponding monomer and subsequent bulk radical polymerization. Different types and amounts of MMT were used, including a sodium and several organomodified clays under the trade names Cloisite Na + and Cloisite 15A, 25A and 30B. Reaction kinetics was measured gravimetrically and the nanocomposites formed were characterized with X-ray diffraction (structure), gel permeation chromatography (molar mass distribution) and differential scanning calorimetry (glass transition temperature). Thermal degradation characteristics were measured with thermogravimetric analysis. Additional experiments with styrene as monomer were carried out in order to make clear the effect of the nanofiller on the polymerization kinetics. It was found that the presence of the bulk ammonium salt used as the organic modifier in MMT could influence the reaction kinetics when diffusion-controlled phenomena occur. Reaction rate was slightly enhanced and nanocomposites with improved thermal stability were formed. In addition, the average molar mass and glass transition temperature of the polymer in the nanocomposites were slightly higher compared to the neat polymer.