This paper investigates how the European Union (EU) and online platforms operationalise ‘political advertising’ and ‘transparency’ in a context of ongoing political and policy debates on regulating online platforms. We compare ongoing EU policy initiatives (revised Code of Practice on Disinformation and Regulation on the Transparency and Targeting of Political Advertising) against platform policies and practices (Google, Mastodon, Meta, Microsoft, Telegram, TikTok, Twitter/X) undertaken to moderate political actors and advertising. We argue that the concept of transparency is used as an ‘empty signifier’: meaningful at the political and declarative level but when translated into practice, leads to diverse results. The paper contributes to academic and policy debates on platform responsibility and political advertising transparency in light of the recent European elections and the future implementation of the Regulation on the Transparency and Targeting of Political Advertising.