The interest in blended synchronous learning environments has increased dramati-cally since the COVID-19 pandemic. However, a key challenge is how to simulta-neously encourage online and onsite student participation. Response systems havebeen found to stimulate student participation in classroom and online education set-tings. This study investigates how online and onsite students participate in blendedsynchronous seminars where a response system is being used. The data comprisesobservations of blended synchronous seminars, students’ written reflections, andstudent interviews, all of which were thematically analyzed. It was found that usinga response system encouraged online and onsite students to participate in variousways. Although online students mostly remained quiet, they perceived to engagethrough listening and thinking, participating in the seminars by absorbing informa-tion, and interacting with the content displayed via the response system. The onsitestudents participated vocally and more spontaneously. All students participated inwritten, anonymous, and game-based modes, suggesting that there were differentand complementary ways for students to participate when using a response system,which extended beyond merely talking or chatting. Notably, most students per-ceived the response system crucial to their participation in the blended synchronousseminars. The findings underscore the importance of encouraging student participa-tion in blended synchronous learning environments, highlighting response systemsas effective tools to encourage onsite and, particularly, online student participation.