Selenium is an essential trace element for human and animal health. Numerous studies reported selenium to possess antioxidant effects, anticarcinogenicity and fertility improvements to name a few. The assimilation of selenium into the yeast via the sulfur metabolic pathway creates selenised yeast which is one of the primary sources of selenium supplementation. In this thesis, novel approaches to the extraction and characterisation of commercial selenised yeast products are described. Extraction procedures were developed for selenomethionine determination. These methods involved both enzymatic and chemical extraction approaches and utilised microwave and ultrasonication energy for the purpose of liberating intracellular selenomethionine. Both the enzymatic and chemical extraction methods were subsequently validated for determination of the analyte. A quantitative assay was also developed for the determination of chiral enantiomers of selenomethionine. The method was applied to commercially-available selenised yeast to characterise any differences in the water-soluble extracts of the products. Significant differences were evident, not only from chiral composition but also from the selenocompounds present in the water extract. Further screening of the water extract by HPLC-ICP-MS revealed numerous other selenocompounds. While coupling of HPLC to ICP-MS was sufficient as an investigative screening tool, selenocompound identification was not possible unless a standard of the analyte was available. To combat these issues other mass spectrometric techniques were investigated. The target water-soluble extracts were lyophilised and resuspended to increase analyte concentration and were analysed by liquid chromatography electrospray quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS). Selenium-containing species were confirmed by selenium’s unique isotopic pattern. Determination of the elemental composition and proposed structure of some of these previously unreported selenocompounds was possible due to the accurate molecular mass from the first mass analyser and from MS2 fragmentation analysis. Therefore, LC- ESI-QTOF-MS may be used as a fingerprint tool to make comparisons between commercially-available selenised yeast products.