Tea belongs to those food products that are often adulterated. One of the most commonly occurring adulteration methods is when a tea manufacturer declares the highest tea quality owing to its composition of exclusively young tea leaves & leaf or floral buds, but, in fact, such tea contains older leaves, and, often, stems. Owing to the adulteration of tea, its taste and aroma are wasted. The authors suggested a method of determining crude fibre to verify the origin of raw material. The content of crude fibre is closely connected with the ‘ age’ of leaves. A high content of crude fibre in the product can show that the raw material used is made of older tealeaves (the 5th-6th leaf or older) and of stems. The objective of this paper was to prove whether or not some black teas were adulterated with the addition of older tealeaves and/or with stems. The black teas investigated comprised: teas imported into Poland (and unloaded in the Gdynia Harbour by the Company ‘Port of Gdynia S.A.’) and originating from different tea cultivation regions; teas purchased in the shops located within the Tricity (in Polish: Trójmiasto consisting of the cities of Gdańsk, Grynia, and Sopot). The authenticity of each tea product was determined by measuring the content of crude fibre therein. Based on the investigation results obtained, it was found that the majority of teas imported into Poland, as well as some other teas available on the Tricity market, were teas made of older (qualitatively worse) tealeaves.