The effects of the HF composition, n, in 1-dodecyl-3-methylimidazolium fluorohydrogenate salts (C-12MIm(FH)(n)F, n = 1.0-2.3) on their physicochemical and structural properties have been investigated using infrared spectroscopy, thermal analysis, polarized optical microscopy, X-ray diffraction, and anisotropic ionic conductivity measurements. The phase diagram of C(12)MIm(FH)(n)F (n vs transition temperature) suggests that C(12)MIm(FH)(n)F is a mixed crystal system that has a boundary around n = 1.9. For all compositions, a liquid crystalline rnesophase with a smectic A interdigitated bilayer structure is observed. The temperature range of the mesophase decreases with increasing n value (from 61.8 degrees C for C(12)MIm(FH)(1.0)F to 37.0 degrees C for C(12)MIm(FH)(2.3)F). The layer spacing of the srnectic structure decreases with,. increasing n value or increasing temperature. Two structural types with different layer spacings are observed in the crystalline phase (type I, 1.0 <= n <= 1.9, and type II, 1.9 <= n <= 2.3). Ionic conductivities parallel and perpendicular to the smectic layers (sigma(parallel to) and sigma(perpendicular to)) increase with increasing n value, whereas the anisotropy of the ionic conductivities (sigma(parallel to)/sigma(perpendicular to)) is independent of the n value, since the thickness of the insulating sheet formed by the dodecyl group remains nearly unchanged.