The major components contained in commercial Bis-GMA monomer were isolated by thin layer chromatography (TLC) and identified by NMR and high resolution mass spectroscopy. In addition to the two major components already known (Bis-GMA and Iso-bis-GMA), an unknown hydrophilic third major component was isolated and identified as 2,2-[4-(2-hydroxy-3- methacryloyloxy-1-propoxy)-4'-(2,3-dihydroxy-1-propoxy)]dipheny lpropane. This compound was designated as BIS-GMA-H, because it has a structure of Bis-GMA with one of which methacrylic ester bond hydrolyzed.