Detailed changes in composition of fatty acids, as well as alterations in the content of triacylglycerols and tocopherols during the maturation of Katalo\u144ski hazelnuts grown in Jankowice, Poland were studied. Samples with different maturity levels were collected at weekly intervals between August and September 2013. Nineteen different fatty acids were identified. The amount of oleic acid (n9) was the greatest in all samples, the amount of linoleic acid (n6) acid was second greatest, while the amount of palmitic acid was third greatest in all samples, except during the very first stage of maturation. A decreasing trend in the amount of polyunsaturated fatty acids (PUFA) was observed from early developed to completely maturated hazelnuts (from 30.09 to 10.28g 100g-1 of oil). On the other hand, the amount of monounsaturated fatty acids (MUFA) in hazelnuts increased from early to harvest stage (from 22.03 to 79.17g 100g-1 of oil). Linoleic acid and oleic acid were the most abundant fatty acids located in sn-2 position of triacylglycerols. Different distributions of both acids were reported during maturation. Oleic acid reported a constant distribution in an internal position, whereas linoleic acid content in sn-2 position increased during maturation. Finally, the tocopherol content increased from early to harvest stage and the amount of \u3b1-tocopherol was observed to be the highest among tocols
