A process for treating organic matter to convert it into a product comprises the step of contacting the organic matter with supercritical liquid whereby it reacts to form the product. The liquid may be heated by an external heating medium, or may be heated internally of the process by co-feeding an oxidising agent with the liquid. This agent can be in an amount that is predetermined to control the extent to which the reaction mixture is heated. The heat can be supplied to provide sufficient activation energy for the process reaction to occur with sufficient speed and for the liquid to attain sufficient characteristics to cause the reaction.