Heterochromatin protein 1α (HP1α) plays a central role in the organization of nuclear content and in the regulation of gene expression and chromatin compaction. Recent work has shown that this protein can phase separate into liquid droplets and gels in vitro, properties that have vast implications for the mechanism of heterochromatin formation and regulation in cells. Despite the tremendous amount of interest in this process, the amorphous, dynamic and viscous nature of the gel condensates has precluded high-resolution analysis of the molecular interactions that underlie HP1α transitions. Using magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, here we present for the first time a molecular description of the liquid to gel transition of phosphorylated HP1α. This methodology has allowed us to follow in real time the rigidification of the molecular interaction network during gelation and to identify specific residues that contribute to gel formation. Furthermore, the addition of physiologically relevant chromatin polymers disrupts the gelation process while preserving the conformational dynamics within individual HP1α molecules. Our results suggest an important role for chromatin in determining the material properties of HP1α condensates and in establishing the complex dynamics within heterochromatin compartments. Our methodology can be applied to other protein systems that undergo phase separation and thus provide atomic resolution details of an elusive biological process.