The thermal alteration of blue-green algae (Lyngbya) and blue-green algal chlorophyll has been studied at temperatures ranging from 25 to 300°C for periods ranging from ½ hour to 2,000 hours under both oxic and anoxic conditions. An irreversible series of time- and temperature-dependent color changes was observed in the cell walls, intracellular pigments, and extracellular sheaths of the heated algae. These sequential color changes are comparable to changes during eometamorphism of plant microfossils, especially palynomorphs, and seem similar to pigment changes during maturation of Tertiary organic matter. Inferred degradation pathways of intracellular chlorophyll differ from those reported for extracellular chlorophyll and its derivatives; intracellular pigment apparently become grafted onto cellular macromolecules, a process that presumably occurs also during natural diagenesis. Formation of such complexes could be an important diagenetic process affecting the types and distribution of pigments in sediments and sedimentary rocks.