This dataset contains the results from 100 native (L1) Dutch speakers from Flanders (Belgium). These participants completed a (i) lexical decision task and a (ii) phoneme categorisation task. In the lexical decision task, participants were exposed to the accented speech of one Italian L1 speaker of Dutch who pronounced 40 target words with either canonical productions of the /ɪ/-vowel but ambiguous realisations of the /i/-vowel (e.g., vlinder 'butterfly' as [ˈvlɪn.dər], but diefstal 'theft' as [ˈdi/ɪf.stɑl]), or vice versa. Participants’ comprehension of the target words was measured in terms of word endorsement (i.e. accepting or rejecting target words as real Dutch words) and response time (i.e. the time interval between the end of stimulus presentation and participant response). The phoneme categorisation task, then, was used to verify if the Dutch L1 listeners are able to identify the two phonemes correctly and if they perceive the ambiguous sounds as pronunciation variants of one of the front vowels. If so, the participants are expected to identify the ambiguous vowels in minimal /ɪ/-/i/ words (e.g., bid-bied 'pray'-'bid') predominantly as either /ɪ/ or /i/, depending on whether the /ɪ/- or /i/-words contained ambiguous vowels in the lexical decision task.