Collective and self-diffusion coefficients of sodium taurodeoxycholate (NaTDC) and taurocholate (NaTC) D2O micellar solutions were measured, as a function of ionic strength (0-0.6 M NaCl added) and bile salt concentration, by means of quasielastic light-scattering and H-1 pulse-gradient-spin-echo nuclear magnetic resonance measurements. A data analysis, performed by using a simple model of monodisperse Brownian particles with hydrodynamic interaction and hard-core repulsion, allowed us to separate the effect of micellar size and particle interactions at low ionic strength. An estimation of the hydrodynamic radius and of the Debye-Huckel interaction strength was obtained without extrapolating the data to the critical micellar concentration. A discussion based on a hard-body interacting cylinder model suggests that attractive interactions influence the NaTDC diffusivities at high NaCl concentration. With respect to NaTC, at the same ionic strength, greater effective charge and size characterize the NaTDC micellar aggregates that, moreover, show a more extended growth as a function of NaCl and bile salt concentration. (C) 2003 American Institute of Physics.