The phase behavior of the aqueous cholesteric mesophase composed of potassium N-dodecanoyl-L-alaninate (L-KDDA) was studied in the presence of various cations such as a cobalt(III) complex and cesium ions using multinuclear magnetic resonance and laser and X-ray scatterings. The cobalt(III) complexes used are [CO(NH3)(6)]Cl-3, [Co(en)(3)]Cl-3 (en = ethylenediamine), Delta-[Co(chxn)(3)]Cl-3 (chxn = (R,R)-1,2-cyclohexanediamine), [Co(tn)(3)]Cl-3 (tn = trimethylenediamine), and [Co(phen)(3)]Cl-3 (phen = 1,10-phenanthroline). Transition temperatures from anisotropic to isotropic phases determined by (H2O)-H-2 NMR spectra were appreciably dependent on the kinds and concentrations of the added cations. The NH3-, en-, tn-, and phen-complex ions were more favorable for stabilizing the aqueous mesophase of L-KDDA compared to the chxn-complex ion. The change in the order parameters for the surfactant aggregates was monitored by anisotropic splittings of the H-1 NMR spectra envelope and was compared to structural parameters of the mesophase obtained from the laser and X-ray scatterings. The relationship between the pitch length and the order parameter of the cholesteric mesophase in the phen-complex system and that in the chxn-complex one showed two extreme cases; that is, in the former case the order parameter significantly increased with addition of the complex ion despite the slight change in the pitch lengths while in the latter case the reverse relationship held. The specific behavior of the mesophase can be explained from the unique interactions of these two complex ions with the alaninate surfactant. The X-ray diffraction studies revealed that the mesophase structure tends to be formed when the micelles whose major axes are perpendicular to the capillary one become dominant over the parallel ones.