In this paper we show that interfaces in aqueous phase-separated biopolymer mixtures are permeable for all components present in the system. In spinning drop experiments, droplets of the low-density phase decreased up to 90% in volume over a time span of days to weeks, when inserted in a matrix of the high-density phase. We propose an expression for this change of volume in time in terms of diffusion coefficients of the components. From the magnitude of these coefficients, we conclude that the transfer of gelatin from inside the droplet to the outer phase was the rate-determining step in this process. Since the interfaces are permeable to all components, the properties of the system change in time. Therefore, the spinning drop technique is not an accurate method for the measurement of the equilibrium interfacial tension of these aqueous phase-separated systems.