Reconstructing the original biogeochemistry of organic microfossils requires quantifying the extent of the chemical transformations they experienced during burial and maturation processes.Two distinct stages of evolution with temperature are observed: in a first stage, sporopollenin experiences dehydrogenation and deoxygenation simultaneously (below 500 ?C); in a second stage (above 500 ?C) an increasing concentration in aromatic groups and a lateral growth of aromatic layers are observed.With increasing heating duration (up to 900 min) at a constant temperature (360 ?C), oxygen is progressively lost and conjugated carbon-carbon chains or domains grow progressively, following a log-linear kinetic behavior.Yet, performing such laboratory experiments provides key insights on the processes transforming biogenic molecules into molecular fossils.