Increasingly, students are expected to write about mathematics. Mathematics writing may be informal (e.g., journals, exit slips) or formal (e.g., writing prompts on high-stakes mathematics assessments). In order to develop an effective mathematics-writing intervention, research needs to be conducted on how students organize mathematics writing and use writing features to convey mathematics knowledge. We collected mathematics-writing samples from 155 4th-grade students in 2 states. Each student wrote about a computation word problem and fraction representations. We compared mathematics-writing samples to a norm-referenced measure of essay writing to examine similarities in how students use writing features such as introductions, conclusions, paragraphs, and transition words. We also analyzed the mathematics vocabulary terms that students incorporated within their writing and whether mathematics computation skills were related to the mathematics vocabulary students used in writing. Finally, we coded and described how students used mathematics representations in their writing. Findings indicate that students use organizational features of writing differently across the norm-referenced measure of essay writing and their mathematics writing. Students also use mathematics vocabulary and representations with different levels of success. Implications for assessment, practice, and intervention development are discussed.