Specially synthesized silica colloidal spheres with fluorescent cores were used as model electrorheological fluids to experimentally explore structure formation and evolution under conditions of no shear. Using confocal scanning laser microscopy we measured the location of each colloid in three dimensions. We observed an equilibrium body-centered tetragonal phase and several nonequilibrium structures such as sheet-like labyrinths and isolated chains of colloids. The formation of nonequilibrium structures was studied as a function of the volume fraction, electric field strength, and starting configuration of the colloid. We compare our observations to previous experiments, simulations, and calculations. (C) 2000 American Institute of Physics. [S0021-9606(00)70708-2].