We have studied the effect of a supercritical hydrothermal process on the structural and surface morphological properties of synthetic smectite clay, stevensite (ST), in terms of the particle size, in order to enhance the functionality of the synthetic smectites as an inorganic filler for transparent clay/polymer nanocomposites. The ST aqueous suspensions were treated in a flow reactor system at 673 K and 25 MPa. The structural characterizations revealed that the ST retained a layered structure composed of polymeric sheets of SiO4 tetrahedra after the treatment. The treated ST possessed a particle size of 71 nm, approximately twice that of the original ST (36 nm) for the 0.1 wt.% suspension using an operation condition at a flow rate of 0.085 g s(-1). SEM observation revealed that an enlarged particle was formed from cohesive aggregates, suggesting that the increase in size of the particles was caused by the cohesion of the microcrystallites or primary particles of ST. The treated ST was subsequently used to prepare nanocomposites with carboxymethylcellulose sodium salt (CMC Na) to evaluate the effect of the supercritical treatment. The treated ST nanocomposite films retained their transparency which is very similar to the original ST nanocomposite films. Furthermore, the nanocomposite films, which had a high CMC Na ratio ranging from 40 to 90 wt.%, showed improved oxygen barrier properties when compared with those of original ST. The tortuous model revealed that this improvement was mainly due to the increase of the particle size. Consequently, the supercritical treatment successfully brought about the growth of the ST particles, leading to the development of functional synthetic clays for clay/polymer nanocomposites. (C) 2010 Elsevier Inc. All rights reserved.