The anionic speciation of chlorostannate(II) ionic liquids, prepared by mixing 1-alkyl-3-methylimidazolium chloride and tin(II) chloride in various molar ratios, χ(SnCl2), was investigated in both solid and liquid states. The room temperature ionic liquids were investigated by (119)Sn NMR spectroscopy, X-ray photoelectron spectroscopy, and viscometry. Crystalline samples were studied using Raman spectroscopy, single-crystal X-ray crystallography, and differential scanning calorimetry. Both liquid and solid systems (crystallized from the melt) contained [SnCl(3)](-) in equilibrium with Cl(-) when χ(SnCl(2)) < 0.50, [SnCl(3)](-) in equilibrium with [Sn(2)Cl(5)](-) when χ(SnCl(2)) > 0.50, and only [SnCl(3)](-) when χ(SnCl(2)) = 0.50. Tin(II) chloride was found to precipitate when χ(SnCl(2)) > 0.63. No evidence was detected for the existence of [SnCl(4)](2-) across the entire range of χ(SnCl(2)), although such anions have been reported in the literature for chlorostannate(II) organic salts crystallized from organic solvents. Furthermore, the Lewis acidity of the chlorostannate(II)-based systems, expressed by their Gutmann acceptor number, has been determined as a function of the composition, χ(SnCl(2)), to reveal Lewis acidity for χ(SnCl(2)) > 0.50 samples comparable to the analogous systems based on zinc(II). A change of the Lewis basicity of the anion was estimated using (1)H NMR spectroscopy, by comparison of the measured chemical shifts of the C-2 hydrogen in the imidazolium ring. Finally, compositions containing free chloride anions (χ(SnCl(2)) < 0.50) were found to oxidize slowly in air to form a chlorostannate(IV) ionic liquid containing the [SnCl(6)](2-) anion.