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Three dissymmetric gemini surfactants (abbreviated as 12−s−14) in which n-dodecyldimethylammonium bromide and n-tetradecyldimethylammonium bromide are connected at the polar headgroups by a flexible −(CH2)s− spacer (s = 2, 6, or 10) have been synthesized. The influence of the spacer length on the structural and thermal properties of 12−s−14 surfactants was investigated by means of IR and NMR spectral analysis, X-ray diffraction, thermogravimetry, differential scanning calorimetry, and polarizing optical microscopy. Geminis with s = 2 or 10 form monolayers in which two alkyl chains are in the trans configuration, while the gemini with s = 6 forms interdigitated bilayers with two alkyl chains in the cis configuration with respect to the spacer. All compounds exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the liquid crystalline phases of smectic type. The number of thermal phase transitions and the sequence of phases are markedly affected by the spacer length; that is, they depend on the configuration of the two alkyl chains with respect to the spacer.