This paper uses an extensive collection of historical surveys that have only recently been made available to researchers to examine the formation of British attitudes towards the then European Economic Community. It demonstrates thatup until 1967the demographic predictors of support for UK membership were unstable. Thereafter, coinciding with the second UK application, these attitudes started to stabilise and harden, with support becoming highest among men, the youngest age cohorts, the middle class and those with greater education. The renewed politicization of the issue in 1967 also coincided with Labour voters becoming significantly more likely to support membership. Following the change in the parties' positions, Labour voters subsequently become substantially less supportive than Conversative voters. The paper thus supports existing analyses on the role of elite cues, while providing new, robust evidence of the change in demographic associations over this formative period.