This work studies the effect of two different organically modified montmorillonites on several properties as well as biodegradation trends of poly(lactic acid)-PLA- and poly(ε-caprolactone)-PCL- in compost. The used clays showed an average good dispersion into both polymer matrices, whilst the highest mechanical improvements were reached depending on clay nature and/or dispersion. These improvements were considerably higher in the case of PLA based nanocomposites, associated to a higher polymer/filler chemical affinity. Concerning to the biodegradation of PLA and PCL based nanocomposites, it was verified that nanoparticles can give different effects on the polymer degradation trend and depending on: degradation mechanism of neat polymer and clay dispersion level in the polymer. It was found that, when the degradation process of neat polymer takes place through a bulk mechanism (case of PLA), the addition of nanoclays tends to increase the polymer degradation rate because of the presence of hydroxyl groups belonging to the silicate layers, especially for the highest dispersed clay. Contrarily, when the neat polymer degradation process occurs through a preferentially surface mechanism (case of PCL), highly dispersed nanoparticles can partially delay the polymer degradation rate probably due to a more difficult pathway for microorganisms in order to attack the polymer ester groups; this last phenomenon being more evident for the most dispersed clay.