Oligomers of lactic acid (LA) at different concentrations were melt-mixed with a polymeric matrix of high molecular weight polylactide (PLA) to improve their barrier properties. Purified and unpurified LA oligomers, obtained by a melt polycondensation reaction, were blended with PLA in order to study how the purification process affected the final film properties. In all the developed blend compositions, significant improvements in the permeability to both water and oxygen were observed in comparison with pure PLA, achieving permeability reductions of up to 54 %. This behaviour was ascribed to an antiplasticization phenomenon of the barrier properties related to the occupancy of the so-called Langmuir-free volume sites by the oligomer molecules. The results also showed that the permeability of these materials was greatly influenced by the presence of moisture, especially in the blends with unpurified oligomers. This behaviour was mainly ascribed to the higher content of the hydroxyl groups present in the latter materials, which led to an observable water sorption and hence to an increase in free volume for permeation.