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Two different types of calcium fluoride particles (∼325 nm), one of them surface modified using a long-chain organophosphorous reagent, were incorporated into a poly(ethylene terephthalate) (PET) matrix. The CaF2 particles were synthesized by a simple chemical precipitation method. To modify the particles surface, a heat treatment using Cyanex® 921 [tri-n-octylphosphine oxide (TOPO)] dissolved in isopropanol, was carried out. Therefore, unlike the as-synthesized particles, the modified particles contained an amount of TOPO. The composite materials were prepared by melt-blending PET and particles at different filler loadings. The influence of the particles surface modification on the nonisothermal crystallization behavior of PET was investigated by using differential scanning calorimetry and field emission scanning electron microscopy. The Jeziorny-modified Avrami equation was applied to describe the crystallization kinetics and several parameters were analyzed (half-crystallization time, Avrami exponent, and rate constant). According to the results, the fluorite particles act as nucleating agents, accelerating the PET crystallization rate. However, the effect on the polymer crystallization rate was more noticeable with the addition of the nonmodified particles where the surface might play an important role for epitaxial crystallization, while the addition of the particles, with an organic coating layer on the surface, resulted in a crystallization behavior more similar to the observed for neat PET. POLYM. ENG. SCI., 54:2938–2946, 2014. © 2014 Society of Plastics Engineers