Multiwall carbon nanotubes (MWCNTs) and glycidyl polyhedral oligomeric silsesquioxanes (GPOSS) are common additives to enhance electrical conductivity and flame resistance of polymers, respectively. Yet, these additions may appreciably influence their mechanical behavior. In the present work, the synergistic effect of the addition of MWCNTs and GPOSS on the mechanical behavior of multifunctional polymers subjected to several types of quasistatic loading was investigated. The results were discussed, supported by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analyses. A significant increase in the tensile strength of the polymers filled only with MWCNTs is observed as compared to the unfilled material. On the other hand, all other properties investigated, namely, compression, flexural as well as G(IC) fracture toughness properties were degraded. The incorporation of GPOSS into the polymer further deteriorates the mechanical behavior of the filled material. SEM analysis has revealed MWCNTs agglomerations of the order of 100 mu m, while EDS analysis has revealed some areas of incomplete dissolution of the GPOSS into the resin. The results underline the sensitivity of the mechanical behavior of the multifunctional polymers on the dispersion features of the additives and the significance of both CNT agglomerates and GPOSS aggregates for the observed mechanical behavior. (C) 2016 Society of Plastics Engineers