Plasma electrolytic oxidation is used to prepare corrosion- and wear-resistant coatings on light metals. The extensive literature reports on coatings formed under a wide range of different electrical regimes and in diverse electrolyte compositions. However, little work is available that investigates systematically PEO of titanium under a range of electrical variables in a particular electrolyte. In the present work, coatings are formed in a silicate electrolyte under a range of current densities, duty cycles and rates of positive to negative current density. The coatings were found to contain anatase, rutile and amorphous silicate-rich material, with comparatively minor influences of the PEO parameters. Further, coatings were limited in thickness to 40 μm due to a decrease in voltage and intensity of sparks at longer treatment times. The coatings were relatively soft with poor wear- and corrosion- resistances, and a high coefficient of friction although the last could be reduced by incorporation of PTFE particles into the coating. The study also investigates coatings formed in aluminate-phosphate based electrolytes, which generated wear-resistant and corrosion-resistant coatings of increased hardness. A focus was on the use of high-resolution electron microscopy, which has not been reported previously, to determine the details of the coating composition and structure. The findings revealed the distribution of coating species, showing an aluminum-rich outer layer and a titanium-rich inner layer, with phosphorus enriched in a band near the base of the coating. However, the coatings also revealed highly localized variations in composition within their noanocrystalline structures, due to the melting and rapid solidification of the coating material. The study also examined the role of electrolyte purity on the formation and properties of the coatings, which has not been examined elsewhere. Importantly chlorine species from the lower purity electrolyte were shown to enrich near the substrate, resulting in a cracked interfacial layer and reduced adhesion of the coating. Such observations may account for reports of poor coating adhesion in the literature. Further, a reduced purity of the electrolyte results in an erratic voltage response, due to cycles of mechanical breakdown and healing of the coating, with high levels of chloride resulting in a highly porous coating. The distributions of phosphorus and chlorine species within the coatings suggest that these species migrate inwards, with chlorine species migrating faster than phosphorus species.