Polymer composites based on poly(methyl methacrylate) (PMMA)/carboxylic acid group functionalized multiwall carbon nanotubes (MWCNT) were prepared by the ex situ and in situ techniques with 0.05% loading by weight. Composite films were fabricated by solvent casting method. Electrical conductivity of the composites as well as of the neat PMMA polymer was measured in the temperature range 333 K to 423 K. Neat PMMA samples prepared by the same method showed complete insulating behavior. Ex situ technique leads to a lower value of percolation threshold. Infrared spectroscopy was used to analyze the effect of functionalization of MWCNT on the interfacial bonding of PMMA and MWCNT. Thermogravimetric analysis revealed that the maximum degradation temperature has been shifted to higher region for in situ composites compared to PMMA itself-and the ex situ composites indicated better thermal stability. X-ray diffraction study of composites also indicates that in situ composites functionalization incorporated MWCNT particles in the polymer chain. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2954-2961, 2011