Employing optical microscopy and statistical analysis, the structure of a model electrorheological (ER) fluid consisting of 7 μm (log-normal distribution) and 27 μm (normal distribution) diameter glass beads in silicone oil was determined quantitatively as a function of electric field and concentration of beads. The arrangement of the beads in the initial structure at zero field exhibited a high contiguity, but showed no significant alignment with respect to the electrodes. Upon application of an electric field, the beads aligned in the direction of the field, E, the degree of alignment increasing rapidly with E to ∼1 kV/mm and then remaining relatively constant. A well-defined chain structure whereby most of the beads resided along chains that bridged the electrode gap first occurred at a field of ∼0.5 kV/mm, following which only slight changes in the structure took place at higher fields. The spacing of the chains decreased with volume fraction of beads; their thickness increased. The spacing-to-thickness ratio was slightly larger than ideal. The mean free spacing of the chain structure was twice that of the beads prior to the application of the field, indicating that the chain structure was formed by the beads moving in opposite directions towards their nearest neighbors. No clear effect of bead size or distribution on the general behavior was detected.