Composite materials consisting of poly(L-lactic acid) and montmorillonite modified to a different extent, using various contents of hexaclecylammonium cation, were prepared by the solution intercalation method. Investigation of the composites' morphology revealed that a surfactant quantity higher than the mineral's cation exchange capacity (CEC) was necessary for the organomodified clay to be dispersed at nanoscale level into the polymer matrix. The surfactant content in organoclay was found to play a major role in controlling the composite's mechanical properties. Thus, although increase of the alkylammonium concentration initially enhanced these properties, even higher concentrations corresponding to higher modification levels had a negative impact to them causing their dramatic deterioration. Observation of the deformed surfaces showed that the deformation process mechanism of the material is directly related to the degree of clay modification. Thermal degradation studies revealed that the intermediate surfactant excess reinforces the thermal stability of the nanocomposite by increasing the onset decomposition temperature. Additionally, the alkylammonium concentration was found to affect the crystallization temperature and the glass transition temperature of the polymer. In conclusion, an ideal balance between thermal and mechanical properties can be obtained at surfactant quantity equivalent to 1.5 times the clay CEC. POLYM. ENG. SCI., 49:1206-1217, 2009. (C) 2009 Society of Plastics Engineers