In this study, we analysed the relationships between word decoding, vocabulary knowledge, meta-cognitive knowledge and reading comprehension in low-achieving adolescents and examined whether the strength of these relationships differed between Grade 7 and 9 students and between monolingual and bilingual students. Tests were administered to 328 students in Dutch prevocational education. Multilevel analyses showed positive effects of vocabulary and meta-cognitive knowledge on reading comprehension that were consistent across grades and across monolinguals and bilinguals. Additionally, we found a positive effect of word decoding for seventh graders, but no effect for ninth graders. There was also a positive effect of bilingualism on reading comprehension when vocabulary differences were controlled. This finding suggests that for bilingual students, reading comprehension is hampered by limited second-language vocabulary knowledge and that these students will profit more from an increase in vocabulary knowledge than their monolingual peers. The results underline the importance of vocabulary and meta-cognitive instruction for low-achieving adolescents.