The thermotropic liquid crystal behavior of n-alkylammonium, N,N-dimethyl-n-alkylammonium, and quaternary N,N-dimethyl-N-cyanopropyl-n-alkylammonium poly(vinylsulfonate)s was investigated using differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. All the hexadecyl and octadecyl derivatives exhibit a smectic B phase at low temperature and a smectic A phase at high temperature. The dodecyl and tetradecyl derivatives of the first two series exhibit a smectic A phase at all temperatures explored, from ambient up to thermal degradation. The dodecyl and tetradecyl derivatives of the third series show a smectic B phase at high temperature and, quite interestingly, an Ia3d body-centered-cubic phase at low temperature. The structure of the cubic phase consists of molecular bilayers bent according to Schoen＇s gyroid minimal surface. Sitting directly on the minimal surface, the polar heads of the molecules cover the same area in the cubic as in the adjacent smectic phase. The transition from cubic to smectic was briefly discussed.