Word order is a core mechanism for conveying syntactic structure, yet interrogatives usually disrupt canonical word orders. For example, in English, polar interrogatives typically invert the subject and auxiliary verb and insert an utterance-initial "do" if no auxiliary is present. These word order patterns result from differences in the underlying syntactic structures; therefore, distinguishing interrogatives from declaratives is crucial for learning the syntax of interrogatives. More broadly, the ability to differentiate sentence types is critical for avoiding errors in syntax acquisition that would occur if interrogatives and declaratives were evaluated as the same type of utterance. Despite the importance of this issue, little is known about when and how infants begin to differentiate sentence types. In this study we exposed one group of 12-month-old infants to auditory passages of polar interrogatives and another group to passages of declaratives. To test sentence discrimination, infants in both groups were played new sets of declaratives and interrogatives in a procedure in which infants controlled how long they listened to each set. The sentences were acoustically modified to remove all prosodic cues that could differentiate them. Regardless of group, infants listened longer to the type of sentence that was different from the type to which they were initially exposed, indicating that they differentiated the sentence types based on lexical distributional patterns. We thus established that, despite only just beginning to produce single word utterances, 12-month-olds are sensitive to word-order properties that differentiate declaratives from interrogatives.