The ability to generate a convincing and persuasive argument with evidence to support a claim is important for participants in a democratic society. Research has revealed a great variety among the analytical frameworks that have been developed to study students’ arguments. Many of these frameworks have limitations such as focusing on either the structure of the argument or the content and/or are hard to use due to its complexity and in some cases more suitable to scientific argumentation rather than informal argumentation on SSI. Accordingly, there is need for a framework that can be used for assessment purposes and that can be used as support for teachers assessment as well as students own practice in order to improve their informal argumentation. The aim of this research is to present a new analytical framework with focus on content and structure as well as the nature of the justifications that can be applied on informal argumentation on SSI. We present this framework by applying it to authentic grade 12-students’ written arguments on a SSI about genetically modified organisms (GMO). The framework consists of several elements and focus on claims and justifications in arguments. The justifications are categorized with regard to three aspects; subjects, pros/cons and knowledge/attitudes. Our hope is that this framework will be fruitful both for future research on informal SSI-argumentation and in school education. The framework can be used as a tool assessing arguments, their complexity regarding both structure and content and consequently to assess the arguments as a whole. The low complexity of the framework also makes it possible for students to use directly as a tool for practicing argumentation on SSI.