Purpose: The purpose of this study was to examine whether sentence combining with an explicit metalinguistic approach in comparison to typical science instruction was effective in improving written expression and understanding of comparison/contrast in science for eighth-grade students who struggle with literacy. Method: Eighty-four eighth-grade students who struggle with literacy participated in this study. The experimental group (n = 36) received the writing intervention of metalinguistic sentence combining (MSC) during their science class for a total of 400 min (20 intervention sessions, 20 min each), while the comparison group (n = 48) participated in their typical science instruction. Total science instruction time was held constant for both groups. All students completed pretests and posttests to determine an increase in (a) syntactic factors of academic science text such as longer sentence length and use of syntactic forms of connectives, targeted connectives, left embeddedness, and agentless passive voice when responding to a science compare and contrast writing prompt; and (b) listing similarities and differences between two science concepts on a graphic organizer. Results: Treatment was effective in improving the experimental group's score in listing similarities and differences between two science concepts on a graphic organizer. There were no significant differences between the two groups in their use of syntactic factors typical of academic text when responding to a science compare and contrast writing prompt. Conclusions: MSC was effective in improving the experimental student's ability to demonstrate understanding of comparison and contrast in science. Modifications to the MSC intervention may yield better results in the experimental group's post-treatment writing in future studies.