This thesis aims to improve the fundamental understanding of the mechanisms and effects underpinning oxygen embrittlement and high-temperature oxidation of titanium alloys. This has been accomplished by using Atom Probe Tomography (APT) and Electron Probe Microanalysis (EPMA) to quantify oxygen segregation on the micro and nanoscale, linking changes in composition, crystal orientation and microstructure to the local mechanical properties using nanoindentation. The current literature reports that oxygen can embrittle titanium alloys by encouraging the formation of the intermetallic α2 phase, but the formation mechanism is not known. In this work, APT has mapped the interaction of oxygen and nitrogen with the α2 precipitates formed in both Ti-7Al model alloys and Ti-64, resulting in the proposal of an α2 precipitate formation mechanism. The interstitial elements are also mapped in detail within these microstructures to answer the question of where oxygen and nitrogen reside. Nanoindentation studies have determined that α2 precipitates are not the dominant hardening mechanism upon ageing, but suggest that oxygen ordering can cause significant hardening, highlighting the importance of controlling the cooling rate during alloy processing. Current models for α case depth prediction often do not incorporate the effect of microstructure. In this thesis, oxygen ingress on an in-service TIMETAL 834 compressor disc has been shown to be affected by microstructure on both the micron and nanoscale, and reinforces the importance of orientation of α/ β interfaces on oxygen diffusion. The relative contributions of oxygen concentration, crystal orientation and microstructure to the hardness of the oxygen-rich surface layer have been measured using a novel analysis protocol. It has been found that the hardening effects of microstructure and crystal orientation are only significant at very low oxygen concentrations, whereas interstitial solid solution hardening dominates by order of magnitude for higher oxygen concentrations. In this thesis, a deeper understanding of the processes of oxygen embrittlement and high-temperature oxidation has been developed through careful mapping and quantification of oxygen and nitrogen and their effect on mechanical properties. This will enable these processes to be better accounted for in decisions about material processing and aeroengine component life predictions.