In this study, we investigated nanocomposites based on a plasticized natural biodegradable matrix. The polymer used was a natural potato starch, and the plasticizers were glycerol and a urea/ethanolamine mixture. A natural and an organically modified montmorillonite were studied. Two series of films containing 6 wt % nanoclays were prepared by a solution/cast process: the first series was based on neat starch, and the second one was based on 20 wt % plasticized starch. For all matrices, a mixture of intercalated and exfoliated structures was formed by the addition of pristine montmorillonite, whereas an aggregate structure was obtained with organoclay. The thermal stability was not significantly influenced by the addition of clays. Water sorption was examined as a function of the matrix and clay hydrophilicity. The significant reduction of oxygen permeability obtained with natural montmorillonite was related to the high dispersion state of this clay. For urea-ethanolamine composites, specific compatibilizer/clay interactions led to an improvement again in the barrier properties.