The influence of the effective particle surface charge density, sigma(e), on the order-disorder phase transition was examined for aqueous dispersions of charged colloidal silica and polymer latex particles (diameters = 0.11-0.13 mu m). The sigma(e) value of the silica particle was continuously tuned by changi ng the concentration of added NaOH. The three-dimensional phase diagram of the order-disorder transition far the silica system was determined as a function of sigma(e), particle volume fraction phi, and salt concentration C-s, by observing iridescence due to Bragg diffraction from the ordered structure, and further by applying an ultra-small-angle X-ray scattering method. With increasing sigma(e), the disordered dispersion became ordered and thereafter reentered into the disordered state. The presence of the reentrant disordered phase at high sigma(e) conditions was observed for ionic polymer latex systems. The reentrant phase transition was not explainable in terms of the Yukawa potential and charge renormalization model.