This study used in situ polymerization to prepare polyethylene terephthalate (PET) nanocomposites incorporating Ethoquad-modified montmorillonite (eMMT), unmodified hectorite (HCT), or phenyl hectorite (phHCT) particles to study the impact of platelet surface chemistry and loading on thermal, mechanical, and gas barrier properties. eMMT platelets reduced the PET crystallization rate without altering the ultimate degree of crystallinity. In contrast, HCT and phHCT platelets accelerated the polymer's crystallization rate and increased its crystallinity. DMA results for thermally-quenched samples showed that as T increased past glass transition temperature (Tg), HCT and phHCT nanocomposites (and control PET) manifested precipitous drops in G' followed by increasing G' due to cold crystallization; in contrast, eMMT nanocomposites had much higher G' values around Tg. This provides direct evidence of eMMT reinforcement in thermally-quenched eMMT nanocomposites. These results suggest that eMMT has a strong, favorable interaction with PET, possibly through Ethoquad-PET entanglement. HCT and phHCT have a fundamentally different interaction with PET that increases crystallization rate and Tg by 11 to 17 degrees C. Water barrier improvement in eMMT nanocomposites agrees with previously published oxygen barrier results and can be rationalized in terms of a tortuous path gas barrier model. POLYM. ENG. SCI., 52:18881902, 2012. (c) 2012 Society of Plastics Engineers