Background: The present study examined patterns and predictors of reading comprehension growth in first language (L1) and second language (L2) readers in the upper grades of primary school. Previous research suggests that L1 and L2 readers differ in their growth trajectories and that differences in language proficiency play a role in this. However, how predictors at different levels of processing influence growth has not been investigated. In particular, the role of unification skills such as syntactic integration and the ability to form a situation model of a text is under-investigated. Method: In a longitudinal study, 63 L1 Dutch and 109 L2 Dutch readers were followed from fourth to sixth grade (mean ages 10 to 13 years). L2 children had a variety of language backgrounds. In Grades 4-6, reading comprehension was assessed. In Grade 4, participants were also assessed on cognitive ability (nonverbal reasoning and working memory), decoding, vocabulary, and unification skills (syntax and situation model building ability). Patterns and predictors of growth were investigated by means of mixed-effects modelling. Results: Reading comprehension growth was predicted by vocabulary and decoding, such that participants with lower decoding or vocabulary scores made more reading comprehension gains. L1 and L2 readers with high vocabulary scores did not differ in their reading comprehension growth. L2 readers with low vocabulary made less reading comprehension gains than L1 readers with low vocabulary. Unification skills predicted initial reading comprehension but not growth. Conclusions: Findings suggest that linguistic skills above the word level predict initial reading comprehension but not growth. Children with lower initial literacy skills caught up on reading comprehension, but this was less true for L2 children. L2 children with high vocabulary, however, did not differ from their L1 peers.