We show that the carboxyl‐functionalized ionic liquid 1‐(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC‐CH2‐py][NTf2] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X‐ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c+=c+) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c+−c+−a− species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen‐bonded ion pairs (c+−a−) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.

Like‐charge attraction is observed for carboxyl‐functionalized ionic liquids. In the solid state, doubly hydrogen‐bonded cationic dimers dominate. In the liquid, these structural motifs are replaced by single hydrogen bonds between like‐ and oppositely charged ions. Combining X‐ray crystallography, IR spectroscopy, thermodynamic methods and DFT calculations allows all types of hydrogen bonding in the solid, liquid and gaseous states of the ionic liquid to be analyzed and characterized. More information can be found in the Research Article by R. Ludwig and co‐workers (DOI: 10.1002/chem.202200949).