In a disaster context, researchers have observed volunteerism since the 1950s, and since then thefield has been under continuous development. However, despite the long tradition and the number of studies carried out, theoretically informed research on emergent groups is largely lacking.The aim of the present study is to investigate, and interpret theoretically, the assembling process,i.e., the formation of emergent groups of outsider volunteers at a disaster site, by means of a casestudy. The assembling process in unofficial disaster response operations occurs as a largely unplanned result of social interaction. However, our results show that such emergence is not onlyspontaneous but preconditioned by the structures of social life as well. Local networks, access toresources and trust were all of significant importance in order to understand the assemblingprocess of disaster volunteers