This paper explores whether and how frames on European Union (EU) issues/policies that are differently politicised by political parties affect public opinion formation. I argue that the magnitude of the framing effects is not independent of the politicisation of the EU issues. However, the concept of politicisation of the EU consists of two main dimensions – salience and polarisation –, which may influence the effect of frames on public opinion formation on EU issues differently. On the one hand, the salience of EU issues implies that individuals are more likely of knowing and having already a stable opinion on them. On the other hand, the polarisation of EU issues indicates individuals which issues are at the centre of party competition and when motivated reasoning should be activated. To test these hypotheses, I use two vignette experiments that manipulate how citizens react to frames in different contexts of party politicisation of the EU during the campaign for the 2022 Portuguese legislative elections. Each experiment covers EU issues/policies that are selected based on their actual level of salience (high vs low) in the national arena, whereas the degree of polarisation is manipulated. In each experiment, the participants read a lead-in treatment on the level of the salience of the EU issues/policies. The remaining treatments in both experiments will vary in terms of the level of polarisation (high vs low) and the type of frames (positive vs negative). The data will be collected among a representative sample of 2000 Portuguese citizens in December.