Abstract: The imidazolium salts 1-ethyl-2,3-dimethylimidazolium chloride, Edimim[Cl], and bromide, Edimim[Br], exhibit negative and positive thermal expansions, as determined by variable-temperature synchrotron powder X-ray diffraction experiments. Both compounds crystallize in the same monoclinic centrosymmetric space group, showing an anisotropic H-bonding network and imidazolium–imidazolium p+–p+ interactions, which have been corroborated by density functional theory studies. The chloride derivative displays a highly anisotropic thermal expansion with a colossal positive coefficient along one direction. Replacement of Cl– by Br– in the same crystal structure produces an increase of the colossal coefficient and induces a biaxial negative thermal expansion. By studying the molecular vibration factors and the H-bonding framework in their crystals as a function of temperature, it was possible to rationalize at the molecular level the mechanism for the observed anomalies in thermal expansion.

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