The work described in this thesis concerns the microbial degradation of 4-alkylpyrocatechols by the bacterium Pseudomonas putida and a mutant of the fungus Aspergillus niger. The aim was to extend current studies on the methylmuconate pathways by exploring the size and shape of the active sites in the dioxygenase and cycloisomerase enzymes in P. putida and A. niger, and the methylmuconolactone methyl-isomerase enzyme in Rhodococcus rhodocrous N75.