In the millimolar concentration domain (typically 1 mM), dioctadecyldimethylammonium bromide and chloride (DODAX, X representing Br(-) or Cl(-) counterions) molecules assemble in water as large unilamellar vesicles. Differential-scanning calorimetry (DSC) is a suitable technique to obtain the melting temperature (T (m)) characteristic of surfactant bilayers, while fluorescence spectroscopy detects formation of surfactant aggregates, like bilayers. These two techniques were combined to investigate the assembly of DODAX molecules at micromolar concentrations, from 10 to 100 mu M. At 1 mM surfactant, T (m) a parts per thousand aEuro parts per thousand 45 A degrees C and 49 A degrees C, respectively, for DODAB and DODAC. DSC and fluorescence of Nile Red were used to show the formation of DODAX aggregates, at the surfactant concentration as low as 10 mu M, whose T (m) decreases monotonically with increasing DODAX concentration to attain the value for the ordinary vesicles. The data indicate that these aggregates are organized as bilayer-like structures.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)