Nanostructured lanthanum (III)-oxide (La2O3) particles were prepared by a polymer complex solution method and further used for the preparation of lanthanum hydroxide (La(OH)3) nanoparticles. The La(OH)3 nanopowder was mixed with glycerol-plasticized maize starch and the effect of the filler on the thermal, mechanical, and viscoelastic properties of the matrix was investigated. It was expected that this nanofiller, which shows an affinity toward OH groups, would strongly affect the physical properties of thermoplastic starch (TPS). The pure TPS and the TPS-La(OH)3 nanocomposite films (with 1, 2, and 3 wt % filler) were conditioned at various relative humidities (RHs) (35, 57, 75, and 99% RH). After conditioning at 99% RH, the pure TPS films exhibited higher affinity toward water than the nanocomposites. Differential scanning calorimetric measurements showed that, due to retrogradation effects, the melting enthalpies of the films increased with increasing RH. Dynamic mechanical analysis revealed that the mechanical properties in the linear range strongly depend on both the humidity conditions and the concentration of the filler. The results also show that La(OH)3 nanoparticles are good reinforcement for TPS films. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012