A new kind of ionic liquid (IL) with strong Bronsted acidity, i.e., a hydronium (H3O+) solvate ionic liquid, is reported. The IL can be described as [H3O+ center dot 18C6]Tf2N, where water exists as the H3O+ ion solvated by 18-crown-6-ether (18C6), of which the counter anion is bis(trifluoromethylsulfonyl) amide (Tf2N-; Tf = CF3SO2). The hydrophobic Tf2N- anion makes [H3O+ center dot 8C6]Tf2N stable in air. The Hammett acidity function (H-0 = -4.4) of molten [H3O+ center dot 18C6]Tf2N, evaluated using the indicator method, is a new record for ILs and indicates strong aciditiy. The findings regarding this proton-condensed solvate IL are of fundamental interest, and will help in the design of media for new acid-base reactions. (C) The Author(s) 2018. Published by ECS.