The research topic in this paper focuses on four categories of ethnicities: local Croats, Bosnian Croats, local Serbs and local returnee Serb migrants in the settlement of Petrinja. The research attempted to identify factors influencing the establishment of borders among ethnic categories and to discover mechanisms through which processes of exclusion took place. The intention was to determine to what extent the sociocultural characteristics of the categories researched condition the relationship towards the Other as a member of a specific ethnic category and influence the establishment of borders between them. The research confirmed that borders between ethnic categories diminish or stretch in accordance with the interactional situation between them and that sociocultural traits, more than ethnic traits, sometimes condition the relationship towards the Other as a member of a specific ethnic category. That was revealed especially in regard to the category of Bosnian Croats whose members the local majority population perceived as Others. Processes of demarcation and establishing borders according to the principle We/Them between local Croats, local Serbs and local return Serb migrants are not based on an ethnic key, although they are linked to such a key, but rather most often derive from the principle “who was on which side during the war”.