We studied the perception of two contrasts by Norwegian learners of English, one similar in the two languages (/t-d/) and one L2-specific (/s-z/). Participants completed an ABX categorisation task, a lexical decision task (LDT) and an MMN task. Results reveal an asymmetry at the behavioural level between the two phonemes of the /s-z/ contrast: performance was worst when having to reject /z/ nonwords, i.e., nonwords created by replacing /z/ with /s/ (e.g., /ˈpɔɪsn/ for ‘poison’). MMN analyses show an asymmetry between /t/ and /d/, with an MMN for deviant /t/ only, in line with previous studies with Danish speakers but not with English speakers. No asymmetry was found for the /s-z/ contrast.