In order to minimize environmental problems related to the generation and disposal of spent mushroom substrate (SMS) from mushroom cultivation activities, this study aimed to evaluate the thermal behaviour of the spent shiitake substrate (SSS) and its resulting biochar. Analyzing biochars produced at a temperature of 350 degrees C (BC350) was found to be that with the highest energy potential due to a reduced amount of resulting ash and nitrogen, and larger amounts of carbon, hydrogen, fixed carbon, as well as a higher calorific value, gravimetric yield, and gravimetric yield factor. SSS and BC350 were used in thermogravimetric and kinetic analyses. Thermogravimetric analyses were carried out at three heating rates, 10, 15, and 20 degrees C/min in an oxidizing atmosphere. Conversion steps relating to water loss and degradation of hemicellulose, cellulose, and lignin have been identified. The kinetic study was carried out by non-isothermal and non-isoconversional Kissinger method. The activation energies obtained were 110 and 136 kJ.mol(-1) for the SSS and BC350, respectively, and are well within the range found for other lignocellulosic materials, such as bagasse and wheat straw. The pre-exponential factor, in the order of 10(8), indicated the presence of complex elements.