Molecularly imprinted, core-shell type boronate affinity sorbents for isolation and detection of sialic acid (SA) isoforms, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), were synthesized. The monomer mixture containing a phenylboronic acid functionalized monomer was polymerized on highly porous silica microspheres 5.5 mu m in size, in the presence of a SA isomer. The selection of highly porous silica microspheres as the core material eliminated the risk of agglomeration during the formation of polymer shell and resulted in the synthesis of a boronate affinity sorbent with a large surface area available for parking of high number of SA molecules. Neu5Ac was extracted from human serum with high isolation yield and successfully determined in the elution medium by LC-MS/MS. The sorbent is a promising stationary phase for precolumn purification of SA isoforms before LC-MS/MS due to its suitable particle size and high porosity.