Electrophoretic deposition of micron-sized polystyrene latex particles on an indium tin oxide (ITO) electrode was studied in situ using optical microscopy. Strong 2-dimensional clustering of the particles on the electrode surface was observed upon application of a potential. The clustering decreases somewhat with increasing salt concentration. Upon reversal of the direction of the field, the clusters broke up. The aggregation on the electrode cannot be explained by DLVO or dipole inter actions. A possible interpretation may be formulated in terms of electro-osmotic flow.