This paper aims to research the influence of mother tongue (Polish) on the perception of a foreign language (Korean) i.e. how vowel sounds that are totally unfamiliar to the listeners are perceived, how the similar sounds are perceived and whether the perception differs according to the phonetic values of the neighbouring sounds. As a result, the degree of the influence of Ll on the vowels of L2 is different in each case and mostly depends on the familiarity of the vowel in question and on the articulatory similarities between the vowels in both languages. The results are as follows; The best perception was observed with Korean /i/ and /a/ (very similar places of articulation in both languages). The worst degree of perception was Korean /(equation omitted)/ that is very unfamiliar to Polish subjects. Vowels that are not so different from the Ll sounds were perceived fairly well. Another important result is that Polish listeners seem to be more sensitive to lip rounding than to the height of the tongue. The role of the neighbouring sounds seems to be of a considerable importance, Depending on the preceding vowel, a sudden drop or rise in the degree of the perception was observed.