Long chain potassium dialkylphosphate salts, from dioctyl to dioctadecyl, were synthesized and their thermal stability was checked by thermogravimetry. Their thermotropic liquid crystalline behaviour was investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Crystalline at room temperature, they were all found to exhibit upon heating a Ia3d body-centred cubic phase followed by a columnar mesophase of hexagonal symmetry. Structures and structural parameters are discussed.