Brownian motion of spherical particulate systems of different materials and surface conditions in dilute aqueous suspensions of various ionic concentrations was studied using photon correlation spectroscopy, zeta potential measurement, and static light scattering measurement. A relation between the ionic concentration and the hydrodynamic diameters of spheres is found. This relation exists as an enlarged particle diameter proportional to the electric double layer. Such size variation is independent of sphere size but dependent on solution conductivity. A model directly relating the shear plane to the measured hydrodynamic diameters and a guide to the proper use of photon correlation spectroscopy in sizing submicron particles are proposed.