Lignin is the most abundant aromatic biopolymer in the environment and performs a structural and protective role in cells of many land plants. Certain basidiomycete fungi (generally called 'white rots') possess the ability to extensively degrade lignin through the use of extracellular enzyme systems, though the exact mechanism remains unclear. The process is of significance from geochemical, soil science and climate modelling perspectives and has industrial applications in both biopulping and bioremediation. This thesis demonstrates that it is possible to study multiple aspects of the white rot lignin degradation process over time in a single model system, and thus link aspects of the process that are generally investigated in isolation. A model system where lignin-containing substrates (wheat [Triticum aestivum L.], common ash [Fraxinus excelsior L.] and Sitka spruce [Picea sitchensis {Bong.} Carr.]) are degraded by the oyster mushroom Pleurotus ostreatus (lJacq: Fr.] Kummer) was developed and on line thermal hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) used to investigate changes in lignin structure. Supporting analyses included fourier transform infrared spectroscopy (FTIR) of lignin and carbohydrate components of substrates, total organic carbon (TOC) determinations, quantification of the fungal biomarker ergosterol and selected plant sterols, plus assays of the fungal enzymes manganese dependent peroxidase (MnP), laccase and ~glucosidase. On angiosperm substrates, selective lignin degradation occurred with lignin oxidation and side chain cleavage continuing throughout the growth of the fungus. The degradation of wheat straw lignin was more extensive than ash lignin. Amounts of ergosterol, increased throughout the degradation process whilst peak enzyme activities were recorded early on. On Sitka spruce wood, only a limited oxidation of lignin occurred and enzyme activities plus fungal biomass remained low. Additionally, by tentatively identifying the products of a Cannizzaro type disproportionation reaction, the thesis provides evidence supporting use of THM when investigating fungal degradation of lignin.