We have synthesized two series of gemini diammonium surfactants with chain terminating p-oxycyanobiphenyl (OCB) groups (I) or conventional structures (II). The thermotropic mesomorphic behavior of series II has been examined using optical microscopy, differential scanning calorimetry, and X-ray diffraction. All the compounds form one or more smectic phases, at least one of which resembles the smectic T (S-T) phase reported by Alami et al. (Liq. Cryst., 1993, 13, 201), where the ionic headgroups form an ordered array, but the chains have some disorder. The exact chain packing differs from that of the S-T structure. The lyotropic mesophases formed with water have been investigated for both I and II by optical microscopy and X-ray diffraction (I only). Series II compounds form hexagonal (H-1), intermediate (INT) and/or cubic (V-1) and lamellar (L(alpha)) phases; the size of the spacer group (m) has little influence on this. The OCB compounds all form L(alpha) as the only lyotropic mesophase. These results show that the insertion of the rigid terminal OCB moiety has a much larger influence on mesophase structure (and by implication on micelle shape) than the spacer group for m less than or equal to 6.