This paper starts with a historical-archaeological model of the ethnogenesis of the Batavi in the Dutch river delta. The central hypothesis is that the formation of a Batavian identity group is rooted in the Caesarean frontier policy and is the result of an integration process of a relatively small immigrant group from the east bank of the Rhine with local autochthonous groups. The objective of this paper is to confront this model with a numismatic data set, related to the Lower Rhine'rainbow cups' of the triquetrum type. The phase in which these coinages circulated on a large scale in the Rhine delta-and were probably also struck there-nearly coincides with the historically documented formation of the Batavi. It will be argued that the coinages in question are Batavian emissions. Finally, I shall discuss the role these coins may have played in the process of Batavian ethnogenesis, and what they can tell us about the changing socio-political organisation of the earliest Batavian polity.