This article gives the results of a study among 685 students in grades 6, 8 and 10 in the Netherlands to whom we administered grade-appropriate measures of reading comprehension and vocabulary knowledge in their native language (NL), Dutch, as well as, in grades 8 and 10, in English as a foreign language (EFL). All students answered the same questionnaire on (4 components of) metacognitive knowledge of reading. The aim was to explore the relative contributions to NL and FL reading comprehension of a language-specific predictor (vocabulary knowledge) and of general metacognitive knowledge. Results of analyses of covariance structure show that metacognitive knowledge was not entirely implicated in vocabulary knowledge. For older students (grades 8 and 10), metacognitive knowledge appeared to play a significant role in both NL and FL reading comprehension. These findings suggest that, in the educational setting investigated, poorer students in grades 6 to 8, not having spontaneously acquired metacognitive knowledge of reading, stand a good chance of profiting from instruction in this type of knowledge. Furthermore, we found evidence for the so-called threshold hypothesis, according to which (metacognitive) knowledge of reading strategies, reading goals and text characteristics cannot compensate for a lack of language-specific knowledge if the latter remains below a certain threshold level. The limited FL knowledge "short-circuits" the transfer of reading skills to the FL.