This study examined how different types of computer-mediated communication (CMC) influences the way pre-university students argue about genetically modified organisms. A total of 39 dyads discussed the topic using either synchronous (chat) or asynchronous (discussion board) CMC, after which they collaboratively wrote an argumentative text in a synchronous groupware environment. It was hypothesized that synchronous CMC would stimulate deep argumentation because of feedback immediacy while asynchronous CMC would stimulate gathering arguments because it allows increased reflection time. Finally, the study sought to determine if students who argue well during a discussion also wrote better argumentative texts. The results obtained partly confirmed the expectations. Students using synchronous CMC argue in a more elaborated way than students using asynchronous CMC. However, in contrast to the hypothesis, students using asynchronous CMC produced more accurate argumentative texts. This study sheds light on how synchronous and asynchronous CMC will be suitable for specific collaborative learning processes.