Textual plagiarism is a serious violation of established academic protocols, but it requires considerable writing experience and care to avoid as well. Although student understanding of textual plagiarism and their plagiaristic behaviour in English as a Second Language (ESL) and English as a Foreign Language (EFL) contexts have been quite well researched, few studies have sought to evaluate instructional interventions specifically designed to reduce plagiarism by empowering student writers with a better knowledge of plagiarism and skills at source referencing. To the best of our knowledge, no study to date has ever systematically assessed students' understanding of plagiarism and their source referencing performance in response to intervention. This classroom-based research, at a university in Beijing, aimed to discover whether a 6-hour block of instruction could facilitate better understanding of plagiarism and appropriate source referencing skills. The results showed that the intervention did generally give students a better appreciation of how textual plagiarism looks and significantly reduced blatant and subtle plagiarism in their writing. However, students' heavy reliance on original source language did frequently reoccur in student writing if in less clear-cut ways. Some helpful lessons were drawn from this study.