Transport properties of molecules dissolved in room-temperature ionic liquids are highly sensitive to the charge carried by the molecule because of complex ion-ion interactions that could be tuned by addition of a cosolvent. In this connection, the one-electron reduction of oxygen was used as a probe system for studying the effects of the addition of a cosolvent such as dimethylformamide (DMF) into a pure ionic liquid (triethylbutylammonium bis(trifluoromethylsulfonyl)imide) ([Et3BuN[NTf2]) on the diffusion of charged species versus neutral species. Experimental data about the diffusion coefficients of O-2 (Do(2)) and O-2(center dot-) (Do(2)center dot-) and their ratios (gamma = Do(2)(center dot-)/Do(2)) were extracted using scanning electrochemical microscopy (SECM) in transient mode as a function of the DMF concentration. The ratio gamma and both of the diffusion coefficients Do(2)(center dot-) and Do(2)(center dot-) were found to increase exponentially with the DMF volume fractions following the same general tendency described for the viscosity. However, Do(2)(center dot-) varies on a much larger range than Do(2) (around 1000 times more), and O-2(center dot-) retains an almost "pure ionic" behavior for higher DMF fractions. All of these results support the occurrence of a sharp transformation in the bonding character of the RTIL cation upon addition of a molecular solvent, as predicted in recent theoretical simulations.