Composition and existence range of aggregates formed by sodium taurodeoxycholate in aqueous micellar solutions were studied. Electromotive force measurements provide the free concentration of hydrogen and sodium ions. Solubility measurements of barium taurodeoxycholate give that of taurodeoxycholate ions. Experimental data obtained at 25 degrees C and at seven different concentrations of N(CH3)(4)Cl, used as an ionic medium, can be explained by assuming the presence of aggregates with different composition depending on the concentration of the reagents and the ionic strength. The comparison with deoxycholate and glycodeoxycholate shows analogies and differences. Protonated species were observed only at pH approximately less than 8.3. At pH’s within the range 8.3-10 the distribution of the micellar aggregates remains nearly constant at a given ionic medium concentration. As it is expected, the size of the micellar aggregates increases upon increasing the ionic strength. A trimer was found for all the concentrations of the ionic medium. Most of the species found have aggregation numbers of the anion that are multiples of 3. It can be supposed that the trimer may be the fundamental unit of the structure of the micellar aggregates. An appreciable amount of a dimer, if any, was not detected. The affinity of sodium ions for the micellar aggregates is greater than that of N(CH3)(4)(+) ions. The charge density of the micellar aggregates was determined to be low.