This study intended to examine Swedish upper secondary school students' argumentation related to the socioscientific issues about sustainable consumption and production, where the students take a position more specifically about a vegetarian diet. Examining sustainable development as an SSI becomes natural here as it is a constantly emerging feature in both the media but also within the school where it is used to give students a developing knowledge to be able to make informed decisions for future problems. This gave the idea to sustainable consumption and production which is one of the sub-goals (goal 12) of the UN goals for sustainable development which can be linked to the question of whether expanding vegetarian diets in schools would be better for sustainable development. In order to understand students' informal argumentation, the choice is to use the SEE-SEP model, which was used as a tool that gives the possibility to categorize arguments and gain an understanding of which grounds the arguments were based upon. The study will also examine the students' development of arguments during the course of the study to examine if there are changes to the argumentations during the study. The results show that students chose to mainly use environment/ecology and science-based arguments during the course of the study. An overview of the arguments shows that the students choose to use the knowledge aspect as their main support when it comes to the design of the arguments. The process of the study shows that there are changes in the design of the arguments and which categories are chosen to build the arguments.