The temperature-versus-composition phase diagrams of eight different 1-alkylpyridinium octane-1-sulfonates (APOSs) in water were studied by H-1 NMR, H-2 NMR, pulsed gradient spin-echo NMR, small-angle X-ray diffraction, differential scanning calorimetry, surface tension and conductivity measurements, and polarizing microscopy. The number of carbons (n(c)) in the hydrocarbon chain of the pyridinium counterions was varied from n(c) = 1 to n(c) = 8 to study how the phase behavior of the APOS/(H2O)-H-2 systems was affected by a change in the chain length of the counterion. The sodium octane-1-sulfonate (NaOS)/water system was used as a reference. This system formed an isotropic micellar solution (L-1) phase and a normal hexagonal (H-I) phase. All APOSs were readily soluble in water and formed L-1 phases. The surface tension above the critical micelle concentration for nc between 1 and 3 was higher than that for NaOS, and it decreased steadily for the different APOSs with increasing chain length. The area per molecule at the air/solution interfaces was rather constant at 68 Angstrom(2) for n(c) between 1 and 7. For 1-octylpyridinium octane-1-sulfonate (OPOS), it was about 5 Angstrom(2) smaller, which was just outside the estimated error. However, the smallest area was obtained for NaOS. At higher surfactant concentrations, liquid crystalline phases formed. Both cubic and H-I phases were found for n(c) = 1 and 2, while for n, between 3 and 5 only an H-I phase was observed. Hi and lamellar liquid crystalline (L-alpha) phases formed for n(c) = 6 and 7. The only liquid crystalline phase found in the OPOS system was a L-alpha phase. The NaOS HI phase was the only liquid crystalline phase that showed a linear relation between the (H2O)-H-2 NMR quadrupolar splitting (Delta(W)) and X-surf/X-W, where X-surf and X-W are the mole fractions of surfactant and water. The OPOS lamellae were found to be much thinner than expected, indicating a defect lamellar structure. This was further supported by the behavior of the quadrupolar splitting of deuterated OPOS. The anomalous behaviors of the 2H(2)O NMR quadrupolar splitting observed in the L-alpha phases of 1-heptylpyridinium octane-1-sulfonate and OPOS were interpreted in terms of changes in the population of the water molecules residing in different sites combined with a continuous rearrangement of the lamellae surface with the possible development of holes. The appearances of the phase diagrams were discussed in terms of surfactant molecular geometry and the packing of the amphiphiles in the aggregates formed.