The effect of several inorganic compounds in the heterogeneous nucleation of poly(ethylene terephthalate) (PET) and poly(caprolactam) (nylon 6) was studied. Six inorganic chemicals were specifically selected, on the basis of their crystal structure, basal lattice dimensions, and thermal properties, in order to study the effect of temperature in their nucleation capabilities. Considering that temperature is one of the most important variables involved during processing of thermoplastics, the impact of the nucleation ability of the intentionally added heterogeneities was studied in terms of thermal conductivity, thermal expansion, and heat capacity. Six inorganic chemicals, which crystallize in the cubic system, were selected considering that they had variations in one of the thermal properties while the others remained practically constant along the temperature range of interest. The results indicated that, at constant surface area, chemical compounds with lower overall values of thermal expansion and also those with overall higher values of heat capacity promote heterogeneous nucleation; the effect of heat capacity was, however, also found potentially related to a different intrinsic mechanism. The impact of thermal conductivity was not considered significant. (C) 2003 Wiley Periodicals, Inc.