The side products in a ring polystyrene made by anionic polymerization was studied by liquid chromatography at the chromatographic critical condition (LCCC) and matrix-assisted laser desorption desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The linear PS contaminants and dimeric ring PS from the monomeric rings were seperated by LCCC from the ring PS. The chain structure was identified by MALDI-TOF MS.