Coprocessing behavior of mixtures of high density polyethylene ͑HDPE͒ and deoiled cakes of Jatropha and Karanja was studied by thermogravimetric analyses under dynamic conditions in the presence of nitrogen atmosphere and compared with those of individual materials. Experiments were carried out in the temperature range of ambient temperature to 900°C at two heating rates ͑5 and 20°C / min͒. Kinetic studies indicated activation energies for HDPE decomposition to be 235 and 258 kJ/mol at heating rates of 5 and 20°C / min, respectively. Values of activation energy for pyrolysis of cakes of Jatropha and Karanj and those for cake-HDPE mixtures varied with the rate of heating as well as with the three temperature ranges. This variation has been explained based on the materials' decomposition behavior. Reduction in activation energy for decomposition of the mixtures implies synergetic effects to be existing when two materials are coprocessed together.