There is significant overlap between reading and writing, but no known standardized measure assesses these jointly. The goal of the present study is to evaluate the properties of a novel measure, the Assessment of Writing, Self-Monitoring, and Reading (AWSM Reader), that simultaneously evaluates both reading comprehension and writing. In doing so, we evaluate reliability (Cronbach's alpha) and various aspects of construct-related validity, including separate criterion measures of reading and writing, and the AWSM Reader's relations with other variables, including language and executive function (EF), both of which are implicated for both reading and writing. Participants were 377 3rd, 4th, and 5th graders with or at-risk for reading and writing difficulties. Reliability was low for the AWSM Reader reading comprehension ([alpha] = 0.58), but good for writing ([alpha] = 0.75-0.80). Criterion-related validity indices revealed moderate correlations with other standardized and commonly used reading and writing measures, r = 0.47 to 0.50 (all ps < 0.001). Additionally, validity data supported the relations of both language and EF to AWSM Reader reading and writing, with EF showing unique prediction in both reading and writing domains. Results provide initial support for the measure per se but stress the difficulty in constructing combined reading and writing measures; directions are given for future work. Results also add to data on the contributions of language and EF to both reading and writing.