The formation of polynuclear aromatic hydrocarbons (PAH) dioxins and furans which arise from incinerators and coal-fired combustion systems has been the focus of attention for many years. Some of these compounds are considered to be formed in the combustion region and other in the cooler post-combustion environment. The accurate measurements of trace emissions such as those of dioxins are difficult and expensive; consequently it is useful to examine these from a modelling and thermodynamic point of view in order to make design predictions and derive a full analytical specification from incomplete experimental data. The thermodynamic properties combined with the kinetic pathways have been used to examine the likely routes of formation of dioxins. These processes were modelled using the Sandia National Laboratories code PSR (Perfectly Stirred Reactor). The properties demonstrated that formation of these compounds occurred during the quenching process for the species which were thermodynamically favoured. Proper insight into the chemical features may help to improve incinerator installation so as to reduce, or eliminate, these emissions.