Differential and thermogravimetric analyses of New Zealand modem, Pleistocene (<30 000 years) and Tertiary (>40 000 000 years) Agathis resins show a progressive change in their combustion profiles with increasing age. Differences between modern resins and Pleistocene Kauri gums are minimal. Both exhibit thermal events in the low-temperature range, commencing near 100 degrees C and extending to 300 degrees C, that reflect the relative abundance of more reactive, undersaturated, carboxyl-rich, unpolymerized components in the younger samples. Diagenetic changes accompanying thermal maturation reveal a reduction in intensity of thermal responses below 200 degrees C and a progression in the temperature of the major thermal combustion event, from 350-450 degrees C in the younger resins, to 450-580 degrees C in the 40 000 000-year-old copals. These variations in thermal behavior accord with a reduction in undersaturated bonds and functionalized groups, and an increase in the proportion of aromatized groups that characterize resin aging. Used judiciously, thermal behavior may indicate maturation histories and resin associations.