The elastic modulus of organic matter can strongly influence the mechanical behaviour of source rocks. Although recent advances have shed crucial light on the mechanical properties of natural organic matter under ambient conditions, the elastic properties of kerogen and bitumen at reservoir temperatures remain poorly constrained. In this paper, we use a novel atomic force microscope technique to measure the changes to organic matter during the heating of an organic-rich shale. Our measurements show that bitumen becomes more compliant with heating and in an experiment during which the temperature was increased from 25 to 225°C, the reduced elastic modulus dropped from 6.3 to 0.8 GPa. In contrast to bitumen, we were unable to discern any significant changes to the elastic modulus of kerogen with increasing temperature. Our results suggest that the temperature dependence of the elastic properties could be used as an additional method to differentiate between bitumen and kerogen in shales. Moreover, our analysis indicates that temperature should be taken into account when modelling the elastic properties of bitumen under reservoir conditions.