A homologous series of dialkyldimethylammonium bromides bearing a cyano group at the end of one of the alkyl chains was synthesized. The cyanoalkyl group ranges from monomethylene to hexamethylene and the cyano-free chain from dodecyl to octadecyl. The thermotropic liquid crystal behaviour of the series was investigated using differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. The molar volume of four selected compounds was determined as a function of temperature using dilatometry. The low-temperature crystal phases were found to have a lamellar structure in which the ammonium bromide groups are arranged within the layers in two distinct planes, alternately separated by single layers of upright, interdigitated alkyl chains and double layers of cyanoalkyl chains coupled through the cyano end groups. The high-temperature smectic A phases were found to display a similar structure regarding the layering of the molecules, in which however the alkyl and cyanoalkyl chains have a disordered conformation. The role of the cyano end groups is discussed by comparing this smectic A structure with the known ordered smectic T structure of the cyanofree dialkyldimethylammonium bromides.