We describe a quantitative Fourier transform ion cyclotron resonance mass spectrometric (FTICR MS) analysis of the relative proportions of post-translational modification states (PTMs) of core histones in cultured cells and tissues. A novel preseparation process using a monolithic column interfaced to a 12 T FTICR MS equipped with electron capture dissociation (ECD) yields very high mass accuracy spectra, allowing direct assignment of the PTMs present in the dominant modification states of intact H4, resolving a well recognized ambiguity between trimethylation and acetylation states. By eliminating preseparation, we also obtain a highly quantitative analysis of the distribution of H4 PTMs. Rapid, extensive, and reversible effects on PTMs induced by a histone deacetylase inhibitor indicate that H4 and other core histones are accessible to modification throughout the chromatin, not just in regions of active transcription. These methods provide tools for analysis of the histone code and its role in chromatin function.