The effects of interspace distance and the possible chemical interactions between PLA and the organic modifier of montmorillonites, Cloisite 15A, 20A and 30B on thermal degradation of PLA in the absence and presence of water vapor were investigated by direct pyrolysis mass spectrometry, (DP-MS) in addition to XRD, TEM, DSC, TGA analyses. The DP-MS results clearly showed that the way in which the polymer was incorporated into the nanocomposite strongly depends on the mixing technique, the interspacing between the clay layers and mostly on the type of the organic modifier used. Strong evidences for interactions of the organic modifier of Cloisite 30B with PLA chains resulting in exfoliated structure were detected. In the presence of water vapor, the relative yields of products due to transesterification and cis-elimination reactions were diniinished. Thermal degradation of PLA nano composites was shifted to low temperatures as the partial pressure of H2O was increased. (C) 2016 Elsevier Ltd. All rights reserved.