Aprotic ionic liquids ([C4pyr]+[NTf2]-, [C8mim]+[NTf2]-, [C8mim]+[OTf]-, [C8mim]+[PF6]-, [C8mim]+[BF4]-, [C12mim]+[NTf2]-, and [C12mim]+[Cl]-) were successfully evaluated as viscosity modifiers, emulsion inhibitors, and demulsifiers. They are prone to be applied for the heavy oil production since such crude oil type potentializes some issues that are typically observed in the petroleum industry. Among them, it is worth highlighting the low flowability due to the oil's high viscosity. Also, high-viscous crude oil usually generates stable emulsions since they have high resins and asphaltene contents, which are interfacial-active. Thus, for operational and economic reasons there is a need for technologies that facilitate the improvement of the heavy-oil flowability as well as the demulsification process. Then, this work aimed to analyze the influence of the ionic liquid chemical structure (cationic alkyl length, cationic functional group, and anion type) upon the oil viscosity, emulsion inhibition or demulsification, at concentrations of 5 and 100 ppm (m/m). Two Brazilian heavy-oils were used for this study. Viscosity measurements of these oils at room temperature (25 degrees C) were made by an oscillatory rheometer before and after the addition of ionic liquid. Also, the emulsification inhibition and demulsification efficiency were analyzed by centrifugation. The results showed that the 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide promoted a demulsification efficiency higher than 99%. In turn, the 1-butylpyridinium bis(trifluoromethylsulfonyl)-imide was the one that worked as an emulsion inhibitor. On the other hand, the interaction between the studied oils and some of the used ionic liquids enhanced the oil viscosity. All the evidence points out to the fact that it was caused by the asphaltene aggregation due to the formation of an ionic liquid-asphaltene molecular structure. The observed viscosity increase was of 11% for oil A and 7% for the oil B. (C) 2019 Elsevier B.V. All rights reserved.