This Thesis investigates the production of high-value chemicals, such as vanillin and guaiacol, by the decomposition and oxidation of lignin in high temperature water. Currently, there is significant global interest in developing chemical production methods that utilise biorenewable feedstocks in place of crude oil and natural gas. Lignin, a complex set of biopolymers found in wood, straw and similar plant materials, is a viable biorenewable raw material for the production of aromatic chemicals. However, currently lignin is mostly used as a low-value fuel in pulp mills. Chapter 1 highlights the concerns that have led to the current demand for greater utilisation of biomass. In that respect, the potential uses of lignin are described. Supercritical water is discussed in relation to green chemistry and specifically as a medium for carrying out oxidation reactions on methyl aromatics. The equipment and methodology used for carrying out experiments and the instruments used for product analysis are described in Chapter 2. Preliminary work that was carried out is described in Chapter 3. These experiments look at the stabilities and oxidation of monomeric aromatic aldehydes, acids and phenols in high temperature water. These substrates each contained either a p-hydroxyphenyl, guaiacyl or syringyl unit. The work in Chapter 4 examined the use of metal bromides and hydrobromic acid as catalysts in the oxidation of three lignin model compounds; 2-methoxy-4-methylphenol, 4-ethyl-2-methoxyphenol and eugenol. These reactions were performed in the near-critical to supercritical region of water. Lignin samples from both Kraft pulping and sulfite pulping sources were shown to breakdown to vanillin, vanillic acid and guaiacol through oxidative treatment in a high temperature water continuous-flow reactor. This work is described in Chapter 5. The overall conclusions of this Thesis are summarised in Chapter 6.