Epigenetic regulation is directed, in part, by the correlated placement of histone post-translational modifications, but the mechanisms controlling correlated modifications are incompletely understood. Correlations arise from crosstalk among modifications and are frequently attributed to protein-protein interactions that recruit enzymes to existing histone modifications. Here we report the use of a peptide array to discover acetyltransferase-mediated crosstalks. We show that p300/CBP associated factor (PCAF)/GCN5 activity depends on the presence of a distal arginine residue of its histone H3 substrate. Modifications to H3 Arg8 decrease PCAF acetylation of H3 Lys14, and kinetic data indicate that arginine citrullination has the strongest effect in decreasing acetylation. Mutagenesis experiments demonstrate that PCAF specifically interprets H3 Arg8 modifications through interaction with residue Tyr640 on the surface of its catalytic domain, and this interaction regulates Lys14 acetylation by substrate discrimination. PCAF discriminates modified peptides as well as semisynthetic proteins and reconstituted nucleosomes bearing Arg8 modifications. Together, this work describes a method for systematically mapping crosstalks and illustrates its application to the discovery and elucidation of novel PCAF crosstalks.