The changes in size and microstructure of alkyltrimethylammonium halide (C(n)TAX) micelles as a function of alkyl chain length (even number of carbons between 12 and 18) and NaCl concentration (0-2 M) in aqueous mixtures with and without trihydroxy or dihydroxy bile salts, sodium cholate or sodium desoxycholate (NaC or NaDOC), have been investigated by cryo-TEM, time-resolved fluorescence quenching, and relative viscosity measurements. Without additions, dilute solutions (25 mM) of all the surfactants form globular micelles, with aggregation numbers increasing with the chain length (to 230 +/- 3 in C(18)TAB at 40 degrees C). Addition of NaCl results in a growth of the micelles. For C(16)TA(+) a transition to long threadlike micelles occurred in 2 M NaCl, whereas the micelles remain globular at shorter chain lengths. For C(18)TA(+) a mixture of globular micelles and large structures was observed at 0.5 and 1. 0M NaCl. The pseudoternary C(n)TAB/NaC/NaCl systems showed results dependent on the alkyl chain length. On addition of NaC to C(12)TA(+) and C(14)TA(+), in the presence of salt, a monotonous decrease in aggregation numbers with increasing concentration of NaC is found. For the longer alkyl chains, a micellar growth resulting in a transition to threadlike cylindrical micelles occurs first, giving a peak in the viscosity. The transition is most pronounced in the C(18)TA(+) case. Further addition of NaC give smaller micelles. The various stages in the transition are seen from cryo-TEM results. In the pseudoternary C(16)TAB/NaDOC/NaCl system, the viscosity was orders of magnitude higher than in the corresponding compositions of the NaC system. The structure of the cylindrical micelles depend on the composition and mole fraction of NaDOC. The change in size and structure on a molecular level is discussed in terms of the structure of bile salt anions and its preferred modes of orientation in the mixed micelle under different conditions.