This thesis describes the making of titania nanocrystalline particles (photonic crystals) and their use as photocatalysts. Chapter 1 describes the work that motivates this research - this includes a description of these structures as propagating a 'slow photon' effect that could make them excellent photocatalysts for decomposing organic compounds. Chapter 2 describes the synthetic procedures that make these photonic crystals. The three-step procedure is detailed involving the infiltration of voids of an opal structure, by either sol-gel or CVD with the characterisation of these structures revealed by several procedures. Chapter 3 documents the photocatalysis studies performed on these structures, which involve the use of the stearic acid test. It was shown that stearic acid is decomposed with half-lives as low as 20 minutes. This does compare very favourably with the other forms of titania tested. Actinometry was performed and quantum efficiency was calculated for quantification purposes which compares favourably with values reported elsewhere. The series of studies showed no correlation of activity with thickness of film. These new films were super hydrophilic but also displayed physical frailties. Chapter 4 details work correlating photocatalytic activity to particle sizes, work with controls, dye absorption experiments and BET area measurement. Chapter 5 describes some studies of inverse opals as gas sensors.