A simple method for the determination of the two most abundant sialic acids released from glycoproteins based on CZE-MS is presented. Several parameters like BGE with various organic modifiers and sheath liquids were studied with respect to their suitability for the fast and easy analysis of the selected compounds by CZE-MS. Finally, a BGE containing 10 mM ammonium acetate allowed the quantification of N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) in glycoproteins as well as human plasma samples. LOD and LOQ were 2 mu g/mL and 6 mu g/mL, respectively.