We investigate the collective thermal diffusion of silica charged spheres in Sulpho-Rhodamine B/water solution at different concentrations by measuring time-dependent thermal and Soret lensings. We show a significant concentrationdependence of the thermal diffusion coefficient DT, not previously reported. Moreover, the results clearly show that both mass diffusion and Soret coefficient are collective quantities being strongly dependent on the particles' packing fraction. Our Z-scan setup allows us to investigate the dynamics of the system at low wave vector, which addresses the influence of the interparticle interactions on the thermal diffusion of the colloids. © 2009 American Chemical Society.