Multiwalled carbon nanotubes (MWCNT) were dispersed in presence of various surfactants and introduced into isotactic polypropylene (PP) via melt blending. The effect of the surfactants on the crystallization and mechanical properties was studied on the injection molded specimens by isothermal differential scanning calorimetry and tensile tests, respectively. The results reveal that the presence of surfactants affects the mechanical and thermal properties of the nanocomposite. This information is supported by thermogravimetric analysis of the surfactants to examine their behavior at temperatures relevant for the processing of PP. In addition, they have to be taken into account as a third phase between the MWCNT and polymeric matrix, affecting the crystallization and failure behavior of PP or causing the formation of pores at elevated temperatures. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 2583-2590, 2010