High catalytic activity and a broad substrate range are characteristic of P450 fusion enzymes of the CYP102A class. P450 BM3 (CYP102A1, BM3) is a paradigm for the P450 fusion enzymes and is accredited with the highest monooxygenase activity in the P450 superfamily, a property which has led to its engineering and exploitation for biotechnologically valuable oxidation reactions. Initial research in the thesis focused on characterisation of a novel P450-redox partner fusion enzyme from the thermophilic fungus Myceliophthora thermophila (CYP505A30, P450MT1). Sequence alignments revealed a P450 domain and a diflavin P450 reductase domain with high sequence similarity to BM3’s domains (41% and 31% amino acid identity, respectively). The purified 118 kDa protein is soluble and exhibits characteristic P450 spectral properties, giving a Soret absorption shift to 450 nm upon binding CO to its ferrous heme iron. Binding titrations of intact P450 MT1 and its expressed P450 (heme) domain with fatty acid substrates and imidazole-based inhibitors revealed type I (blue) and II (red) Soret shifts, respectively, typical of other members of the P450 superfamily, and enabled determination of substrate binding constants. HPLC analysis confirmed stoichiometric amounts of bound FAD and FMN cofactors. Subsequent kinetic and biochemical studies included stopped-flow kinetic experiments showing that NADPH-dependent reduction of P450 MT1’s FAD cofactor occurs with a rate constant of ~150 s-1 at 20 °C. P450 MT1 has an unconventional substrate hydroxylation profile for saturated fatty acids. It hydroxylates these substrates predominantly at positions ω-1, ω-2 and ω-3. However, an unusual property of this enzyme is observed in its strong preference (~85% of total converted product) for either the ω-1 or the ω-2 position on odd and even chain length fatty acids, respectively. However, it displays higher selectivity for branched chain fatty acids over straight chain fatty acids, e.g. for the substrate iso-myristic acid, similar to BM3’s properties. Other work done focused on biophysical characterisation of the model P450-reductase fusion enzyme P450 BM3 from Bacillus megaterium. A combination of alternative structural techniques to X-ray crystallography were used to characterise the enzyme. More specifically, electron microscopy (EM) and nuclear magnetic resonance (NMR) were used to gain greater insights into the intimate associations of the enzyme monomers in BM3’s dimeric structure. These studies led to the first structural insights into how P450 BM3’s dimeric complex is organised. Dimerisation in BM3 arises predominantly from self-association of the enzyme’s FAD domains, and wild-type and mutant BM3 FAD domain forms were also characterised. Key FAD domain mutations that prevented intra-/inter-monomer disulphide bond formation facilitated the crystallization and determination of the FAD domain structure, the final part of the BM3 enzyme to have its three dimensional structure resolved. Data reported in this thesis give new insights into the biochemistry of biotechnologically important P450 monooxygenase enzymes from mesophilic and thermophilic microorganisms.