The improvement of separation efficiency for protein analysis in capillary electrophoresis (CE) is a challenging topic in which protein adsorption onto the capillary wall plays a crucial role. In this work, a simple method allowing the quantification of the adsorption of proteins onto the coated or untreated inner surface of the fused silica capillary was developed based on the determination of the retention factor by measuring separation efficiency of individual proteins at different separation voltages (i.e., different linear velocities). This approach was applied to the quantification of the residual adsorption of four test proteins on five-layer polyelectrolyte coatings and bare fused silica capillary. It allows to get a fair ranking of the coating performances toward protein adsorption, whatever their apparent electrophoretic mobilities (migration times) are. Due to the existence of (even low) residual adsorption, the electrophoretic operating conditions (electric field, capillary length, and internal diameter) can be optimized to improve the separation performances resulting in experimental separation efficiency up to ∼600 000 plates.m–1 in conditions compatible with MS coupling. This approach represents a crucial step in the course to get antifouling coatings for protein separation in CE. It can be used for the evaluation and ranking of virtually any coating (neutral or charged) in CE.