Both multiple-choice and constructed-response items have known advantages and disadvantages in measuring scientific inquiry. In this article we explore the function of explanation multiple-choice (EMC) items and examine how EMC items differ from traditional multiple-choice and constructed-response items in measuring scientific reasoning. A group of 794 middle school students was randomly assigned to answer either constructed-response or EMC items following regular multiple-choice items. By applying a Rasch partial-credit analysis, we found that there is a consistent alignment between the EMC and multiple-choice items. Also, the EMC items are easier than the constructed-response items but are harder than most of the multiple-choice items. We discuss the potential value of the EMC items as a learning and diagnostic tool. (Contains 6 figures and 4 tables.)