Olive oil, as a basic component of the Mediterranean diet, has for long been associated with a reduced incidence of chronic inflammatory diseases and cancer as well as longer life expectancy. Due to its positive health effects, olive oil today is synonymous with healthy eating habits. The positive effect is due to the favorable fatty acid composition, primarily oleic acid, as well as biologically active polyphenolic fraction. Among positive effects, their antioxidant activity, anticarcinogenic, immunomodulatory, cardioprotective and antimicrobial effect can be highlighted. The purpose of this diploma thesis was to determine total phenolic, flavonoid and o-diphenol content by UV-Vis spectrophotometry and to determine content of S-(E)-elenolide, oleokoronal and oleomissional by 1H NMR spectrophotometry in selected samples of Croatian olive oils. Total phenolic content was determined using F-C reagent, total flavonoid content using AlCl3 and NaNO2, while o-diphenol content was determined using Na2MoO4. Total phenolic content varies in range 168-628 mg/kg, total flavonoid content in range 102-380 mg/kg and o-diphenol content in range 184-596 mg/kg of olive oil. Based on the comparison of the content of total polyphenols, total flavonoids and o-diphenols, the repeatability of the acetonitrile extraction method was examined. Good repeatability of the method with an error of less than 4% was found. Two extraction methods were compared in the same way. A good agreement was obtained in the flavonoid content, while the polyphenol content of the acetonitrile extract was significantly higher than the methanol extract content. Samples prepared by the acetonitrile extraction process were found to be unsuitable for the determination of o-diphenol with NaMoO4 reagent. Methanol extract has to be analysed immediately after the preparation as the content changes during storage. Content of oleokoronal, oleomisional and S-(E)-elenolide were determined, for the first time in Croatian olive oils. S-(E)-elenolide content varies in range 40-910 mg/kg, oleokoronal content in range of 63- 162 mg/kg and oleomisional cantent in range of 36-169 mg/kg of olive oil.