Electrophoretic analysis of colloidal particles is adversely affected by a host of surface phenomena including electroosmosis, phase wall wetting, and sample or air bubble adsorption. Neutral, hydrophilic polymer coatings significantly control such phenomena on a variety of surfaces. Readily adsorbed poly(ethylene glycol)-poly(ethylene imine) conjugates were found to form coatings whichsignificantly reduce electroosmosis, and positively control adsorption and wetting, in the standard glass sample chambers (5 x 3 x 1 mm i.d.) employed in a representative commercial electrophoresis apparatus (Coulter DELSA 440+). The reduction in electroosmosis was similar to that exhibited by coated quartz capillaries (2 mm i.d.) in a manual apparatus (Rank MK 1). The coatings significantly reduced electroosmosis over a wide range of pH (2 to 11) and ionic strength (1 to 100 mM) and were stable for many weeks under normal laboratory conditions. They greatly enhanced ease of operation and accurancy (sample mean electrophoretic mobility _+ S.D.) of the DELSA 440. Enhanced mobility determination appeared related to reducing electroosmosis gradient flow profiles near the chamber center-axis stationary levels, where particle mobility is typically measured. Such flow profiles also appeared to be affected by asymmetries in the surface properties of DELSA chamber walls. A hydronamic description of electroosmotic fluid flow in rectangular chambers was adapted to analyze the propagation of errors due to nonideal focusing and chamber asymmetries. This analysis indicated that rectangular chambered devices may gain accuracy by measuring particle mobility at stationary levels removed from the chamber center-axes, and that the hydrodynamics of some rectangular chambers may confer accuracy advantages over cylindrical chambers.