In this study, the N-methylpolyvinylpyridinuim polymer has for the first time been used as a silica surface modifier for CE in combination with ESI MS (CE-ESI-MS) The compatibility for ESI-MS was demonstrated by the analysis of peptides and protein digests. The N-methylpolyvinylpyridium surface interacts electrostatically with the ionized silanol groups, giving a cationic surface with a reversed EOF The surface modifier gave rapid and repeatable separations of peptides, proteins and protein digests at acidic pH for more than 4 h of continuous use The CE separation yielded peak efficiencies of up to 4.3 x 10(5) plates/m The surface coating is highly compatible with ESI and facilitates the separation and analysis of complex peptide mixtures as shown by the analysis of BSA digests.