We developed a platform using hydrophilic interaction chromatography and high-resolution tandem mass spectrometry (MS) for analyses of histone H3 that allows comprehensive characterization of 'histone codes' at the molecular level. We identified over 150 differentially modified forms of histone H3.2 in asynchronously grown and butyrate-treated HeLa cells, revealing pervasive combinatorial modification previously unaccounted for by other techniques and providing a clarified estimate of the molecular diversity of histone H3 in mammals.