This article reports on two quantitative studies of English learners' (ELs) interactions with constructed-response items from a Grade 5 state science test. Study 1 investigated the relationships between the constructed-response item-level variables of English Reading Demand, English Writing Demand, and Background Knowledge Demand and the performance of ELs vs. non-ELs on those items. English Writing Demand was the strongest predictor of Differential Item Functioning favoring non-ELs over ELs for constructed-response items. In Study 2, we investigated the student-level variable of English language proficiency level and found that lower English language proficiency was related to greatly increased odds of omitting a response to a constructed-response item, even when controlling for science proficiency. These findings challenge the validity of scores on constructed-response test items as measures of ELs' science proficiency.