Carbonaceous micro-/nanoparticles are efficient infrared attenuation agents which may enhance the thermal insulation properties of polymeric foams by reducing the radiation heat transfer. The effects of low-concentration (<= 0.5 wt %) multiwalled carbon nanotubes and micrographite (mGr) on the extrusion foaming of semicrystalline polyethylene terephthalate were investigated in this study. It is shown that the heterogeneous nucleation induced by the carbonaceous particulates would greatly affect the nonisothermal crystallization behavior during extrusion foaming. The rheology test results revealed that the melt strength of CPET composites was low above 235 degrees C but radically increased when the melt cooled down because of fast crystallization. Consequently, this phenomenon was beneficial to the cell growth and expansion at high melt temperatures because the increased melt viscosity could stabilize the cellular structure during the quenching stage. CPET/carbonaceous particulate composite foams showed high crystallinity, excellent IR absorption, and enhanced thermal insulation properties.