The lyotropic liquid crystal (LLC) behavior of a homologous series of alkyltrimethylphosphonium bromide salts is described. While alkyltrimethylammonium salts are common LLC mesogens that have served in mesoporous materials, phase-transfer catalysis, and polymer-surfactant assemblies, the LLC behavior of the analogous phosphonium salts has been relatively unexplored. Dodecyl- (C12TMPB), tetradecyl-(C14TMPB), and hexadecyltrimethylphosphonium bromide (C16TMPB) were found to exhibit hexagonal, lamellar, and cubic mesophases similar to their ammonium counterparts; however, there is a key difference: Changing to a phosphorus-based headgroup appears to stabilize the cubic phase of C12TMPB and C14TMPB such that the phases extend down to room temperature. Some trends in the lattice parameters of the resulting LLC phases of the alkyltrimethylphosphonium bromides with respect to phase composition and temperature are also discussed. Finally, preliminary use of P-31 NMR spectroscopy, as a method of structural characterization of the phases, showed variation in the peak widths for the different phases but not the expected chemical shift anisotropy seen in the more commonly studied phosphate-based systems; an explanation for this difference is proposed.