Capillaries (25microm I.D.) treated with the double-alkyl-chain cationic surfactant N,N-didodecyl-N, N-dimethylammonium bromide (DDAB) in an improved coating procedure were used for separation of four basic proteins in volatile buffers (ammonium acetate and ammonium hydroxyacetate) as well as in a non-volatile buffer (sodium phosphate) at pH 4. The DDAB coating was stable enough to, without recoating, permit consecutive separations of the proteins up to 9h with good precisions in peak areas (RSD=1.1%) and migration times and with high apparent efficiencies (over 1 million theoretical plates/m) in the presence of a strong anodic electroosmosis. Adsorption of the proteins onto the capillary surface, which in previous studies was found to give a certain contribution to zone broadening, was eliminated with the new modified coating method. Complex formation between the proteins and phosphate buffer was studied and confirmed, and it is proposed that slow protein-buffer component interactions are the main contributions to zone broadening in protein separations by CE.