Nonaqueous capillary electrophoretic separations were performed under high electric field strengths (up to 2000 Vcm(-1)) in ethanolic background electrolyte solution and the contributions of different band broadening effects to plate height were evaluated. Under optimum, conditions, increasing the field strength will provide faster separations and increased separation efficiency Decrease in the separation efficiency at high field strengths was, however, observed in a previous study and now in the present paper an attempt is made to quantify various band broadening effects by applying a plate height model, which included the contributions of the injection plug length, diffusion, electro-migration dispersion, Joule heating, analyte adsorption to the capillary wall, and detector slit aperture, length. Of special interest were the contributions of Joule heating and analyte adsorption to the capillary wall. Poly(glycidylmethacrylate-co-N-vinylpyrrolidone)-coated fused-silica capillaries were used with internal diameters (ID) ranging from 30 to 75 mum. The separation efficiencies obtained: experimentally were compared with the theoretically calculated efficiencies and fairly good agreement was observed for the 30 mum ID capillary. Relatively large deviation: from the predictions of the model was founds for the other capillary diameters especially at higher field strengths. The possible, reasons for the deviation: were discussed.