1-Alkyl-3-methylimidazolium tetrachloropalladate(II) salts ([Cn-mim]2[PdCl4], n = 10, 12, 14, 16, 18) containing a single, linear alkyl-chain substituent on the cation have been synthesised and their behaviour characterised by differential scanning calorimetry, polarising optical microscopy and small-angle X-ray scattering. The salts display thermotropic polymorphism, exhibiting both crystal–crystal transitions and, for n = 14–18, the formation of a thermotropic smectic liquid crystalline phase.