A note on versions:The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher's version. Please see the 'permanent WRAP url' above for details on accessing the published version and note that access may require a subscription. Luxembourg. While these referendums were seemingly distinct from one another, identical campaign posters and arguments were circulated across Europe. Diffusion is a process wherein new ideas and models of behavior spread geographically from a core site to other sites (Bunce et al., 2006). While referendum campaigns have been studied thoroughly, such cross-case influences remain overlooked (e.g. Glencross et al., 2011;Qvortrup, 2006). I show that campaign arguments were not always homegrown in the 2005 EU referendums. Campaigners could learn from the experience of previous campaigns. This was important because strategic arguments blamed the treaty for controversial issues such as degradation of welfare state or loss of national identity. Diffusion across cases was thus crucial in bringing new arguments into the debate. All campaigns were not created equal, and the cases were not independent.The extant literature has not applied diffusion theories to the study of referendum campaigns.Instead of using diffusion processes as an independent variable and trying to trace their effect on the referendum results quantitatively (e.g. Collingwood et al., 2012), I treat diffusion as the dependent variable and detail social mechanisms that connect referendum campaigns through interview data.