In this study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation were evaluated as potential plasticizers and lubricants for polylactic acid (PLA). Both ILs at 5 wt% were well dispersible and partly miscible with PLA as evidenced by scanning electron microscopy, SEM, and glass transition temperature, T-g, characterization, as well as from solubility parameters calculations. However, phase separation in the PLA system containing the IL with tetrafluoroborate anions was observed 1 year after melt processing. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate as confirmed by thermogravimetric analysis. This was translated in corresponding behaviors of the PLA blends. The effects of the IL containing the tetrafluoroborate anion were more pronounced on lubrication as evidenced by lower values of coefficient of friction that could also be correlated with lower contact angle; this IL could then be considered as a more effective, overall, plasticizer than the IL containing the decanoate anion based on reduced flexural modulus and brittleness as well as T-g suppression results. The extent of plasticization, however, could also be related to the molecular weight reduction during melt processing that occurred in the presence of both ILs, but to different degrees. POLYM. ENG. SCI., 50:1105-1110, 2010. (C) 2009 Society of Plastics Engineers