In order to evaluate the factors that influence the spatial behaviour of border agglomeration inhabitants and to relate them to the geopolitical Situation, attention is given here to three divided towns in the Baltic Sea region: Haparanda-Tornio, Narva-Ivangorod and Valga/Valka. The three cases represent different geopolitical and ethnical configurations. Boundary theory is discussed as part of (inter)action theory, with particular emphasis on the homogenizing role of the State in contrast to the (inter)action possibilities of individuals living in a border area. In the Haparanda-Tornio study case, a homogenous population on the Finnish side is compared to a more diversified ethnie grouping in Haparanda, Sweden. Here, immigrants from Finland are seen to have an action space directed towards Finland, while bilingual locals act as bridge-builders. In Narva-Ivangorod, the same Russian ethnic group lives on both sides of the boundary, but geopolitical restrictions hamper local cross-boundary interaction. Valga/Valka, in contrast, is divided according to ethnic criteria. Little motivation for border crossing appears to be given, except for the Russian speaking population, which at the same time has the greatest formal difficulties in crossing.The internal European Union borders, both recent and older, thus allow ethnie and linguistic factors to influence the action and communication space of inhabitants.The EU/non-EU boundary restricts the possibilities of local cross-boundary interaction even in situations where the population belongs to the same ethnie group.