The current understanding of epigenetic signaling assigns a central role to post-translational modifications that occur in the histone tails. In this context, it has been proposed that methylation of K9 and phosphorylation of S10 in the tail of histone H3 represent a binary switch that controls its reversible association to heterochromatin protein 1 (HP1). To test this hypothesis, we performed a comprehensive molecular dynamics study in which we analyzed a crystallographically defined complex that involves the HP1 chromodomain and an H3 tail peptide. Microsecond-long simulations show that the binding of the trimethylated K9 H3 peptide in the aromatic cage of HP1 is only slightly affected by S10 phosphorylation, because the modified K9 and S10 do not interact directly with one another. Instead, the phosphate group of S10 seems to form a persistent intramolecular salt bridge with R8, an interaction that can provoke a major structural change and alter the hydrogen-bonding regime in the H3-HP1 complex. These observations suggest that interactions between adjacent methyl-lysine and phosphoserine side chains do not by themselves provide a binary switch in the H3-HP1 system, but arginine-phosphoserine interactions, which occur in both histones and nonhistone proteins in the context of a conserved RKS motif, are likely to serve a key regulatory function.