The well separated transitions observed on heating synthetic core particles containing poly(dA-dT).poly(dA-dT) have allowed physicochemical studies to define the mechanism of the first, reversible transition. The shape of the particle, assessed by sedimentation velocity measurements, becomes extended at temperatures about 10 degrees C below the optical melting temperature; this change is unaffected by cross-linking the histones into a stable octamer. A portion of the DNA undergoes a transition to a structure more like that of protein-free poly(dA-dT).poly(dA-dT), as judged by circular dichroism measurements, also at temperatures below those necessary for optical melting. Digestion of 5'-32P-labeled particles with S1 nuclease at a temperature partly through the first optical transition demonstrates that the initial phase in melting core particles is separation of about 20 base pairs of DNA at each end of the particle segment from the histone core. Implications of this mechanism for transcription of histone-bound DNA segments in chromatin are discussed.