This study examines the causes for the within-language Stroop superiority effect (WLSSE; larger Stroop effect for words appearing in the response language than in another language). Participants named in their first language (L1) or second language (L2) the colour of colour-words or colour-associated words appearing in L1 or in L2. The WLSSE was limited for colour-words. It was concluded that for colour-words, the cause for the WLSSE is that words appearing in the within-language condition belong to the response set, whereas words in the between-language condition do not. This finding challenges previous suggestions that a nonrelevant stimulus language can be suppressed.