In this paper, we report and discuss the reduction mechanism of pentavalent tantalum salt (TaF5) in a room temperature ionic liquid (RTIL) by coupling two complementary electrochemical techniques: the cyclic voltammetry and the electrochemical quartz crystal microbalance (EQCM). The experiments were performed in the hydrophobic 1-butyl-3-methyl pyrrolidinium bis(trifluoromethylsulfonyl) imide ([BMPyr][TFSI]) RTIL used both as a solvent and as an electrolyte. Four different reduction peaks are present in the obtained voltammogram in the range from 0 V (vs Pt-Fc/Fc(+)) to -3.5 V (vs Pt-Fc/Fc(+)). The existence of the first reduction peak depends mainly on the nature of the substrate of the working electrode and is therefore not related to the reduction of the tantalum salt. The second reduction peak is ascribed to the direct tantalum salt reduction to its metallic state. During this reduction, free fluoride ions are produced in the electrolyte in the vicinity of the electrode surface. These ions can then react with diffusing TaF5 to form TaF5+x-x complexes and thus the reduction peaks occurring at lower potential are referred to the reduction of such complexes. This hypothesis was confirmed by addition of LiF in the electrolyte. (C) 2015 Elsevier Ltd. All rights reserved.