This paper is an attempt to lay bare the logic of the rubber trade across the Kwango river (the eastern border of Angola's Congo district) in the environment of the neighbouring Congo Free State. The paper leaves aside the precise historical description of the movements of the Yaka and the analysis of their significance in the colonial context as well as the details of the adaptation of the Zombo entrepreneurs, who dominated this trade, to the new reality of a colonial frontier. Instead it seeks to explain why the rubber trade was at the root of so much of the turmoil - from the mentioned Yaka migrations to Zombo plans to attack the Bula Matari (agents of the Congo Free State) with the help of the Portuguese, as described in government and missionary papers from both sides of the frontier. How was this trans-border trade conducted and how did it conflict with the interests of the Congo Free State ?