Drawing from multiple theoretical frameworks representing cognitive and educational psychology, we present a writing task and scoring system for measurement of students' informative writing. Participants in this study were 72 fifth- and sixth-grade students who wrote compositions describing real-world problems and how mathematics, science, and social studies information could be used to solve those problems. Of the 72 students, 69 were able to craft a cohesive response that not only demonstrated planning in writing structure but also elaboration of relevant knowledge in one or more domains. Many-facet Rasch Modeling (MFRM) techniques were used to examine the reliability and validity of scores for the writing rating scale. Additionally, comparison of fifth- and sixth-grade responses supported the validity of scores, as did the results of a correlational analysis with scores from an overall interest measure. Recommendations for improving writing scoring systems based on the findings of this investigation are provided.