The interest in blended synchronous learning environments has increased dramatically since the COVID-19 pandemic. However, a key challenge is how to simultaneously encourage online and onsite student participation. Response systems have been found to stimulate student participation in classroom and online education settings. This study investigates how online and onsite students participate in blended synchronous seminars where a response system is being used. The data comprises observations of blended synchronous seminars, students' written reflections, and student interviews, all of which were thematically analyzed. It was found that using a response system encouraged online and onsite students to participate in various ways. Although online students mostly remained quiet, they perceived to engage through listening and thinking, participating in the seminars by absorbing information, and interacting with the content displayed via the response system. The onsite students participated vocally and more spontaneously. All students participated in written, anonymous, and game-based modes, suggesting that there were different and complementary ways for students to participate when using a response system, which extended beyond merely talking or chatting. Notably, most students perceived the response system crucial to their participation in the blended synchronous seminars. The findings underscore the importance of encouraging student participation in blended synchronous learning environments, highlighting response systems as effective tools to encourage onsite and, particularly, online student participation.