Socio-scientific issues (SSI) have proven to be suitable contexts for students to actively reflect on and argue about complex social issues related to science. Research has indicated that explicitly teaching SSI argumentation is a good way to help students develop their argumentation skills and make them aware of the complexity of SSI. However, assessing the quality of students' arguments on SSI is evidently difficult for many teachers. This article aims to facilitate teachers' assessment of the quality of students' arguments on SSI by introducing a new assessment framework that represents a low degree of complexity and exemplifying it by applying it to students' written SSI argumentation concerning genetically modified organisms (GMO). The new assessment framework considers both the quality indicators presented in the research literature and curricular guidelines for the science courses in Swedish secondary and upper secondary school. The framework focuses on both the content and the structure that can be revealed in students' SSI argumentation and is meant to function as a tool for identifying quality indicators that could serve as the basis for grading.