The spectroscopic properties of a J-aggregating cyanine dye in the presence of betainic N-n-alkyl-N,N-dimethylammonio acid bromides with alkyl chain lengths between 10 and 16 carbon atoms are examined. In contrast to the usually observed deaggregating influence of micelle-forming agents, an increase in absorptivity and a decrease in J-band width are established. Furthermore, the spectroscopic properties of the J-band depend on the N-n-alkyl chain length of the surfactant. The coupling size of the aggregates increases in surfactant solution as compared to that in aqueous solution. The effects are attributed to a new type of supramolecular structure based on J-aggregates with enclosed monomer molecules of the surfactant.