The current study examined the occurrence and benefits of linguistic alignment in two modalities, face-to-face (FTF) and synchronous computer-mediated communication (SCMC), focusing on stranded prepositions in relative clauses. It further examined how learner characteristics (i.e., working memory, language proficiency, previous knowledge of the target structure) mediate the effects of linguistic alignment. Ninety-four Korean students were assigned to one of the following groups: FTF alignment, SCMC alignment, FTF control, and SCMC control. The alignment experimental groups completed two alignment sessions, finished three stranded preposition tests, and carried out a running span test and cloze test over three weeks. Results indicated not only that linguistic alignment occurred in both FTF and SCMC modes but also that alignment was facilitated significantly more in the SCMC than FTF interactions. Furthermore, the findings suggest immediate and delayed learning effects in both modalities, and that learners' prior knowledge of the target structure was significantly associated with the occurrence of alignment.