In this paper, we show that for aqueous phase-separated biopolymer mixtures (water-in-water emulsions) both interfacial tension and permeability of the interface are important for the relaxation process of deformed droplets. We give an expression for the characteristic relaxation time that contains both contributions. With this description, the interfacial tension and the permeability can be deduced from cessation-of-flow experiments. The results show that for samples that are very close to the critical point the interfacial tension, calculated without taking into account the permeability, are overestimated significantly. For samples close to the critical point, the permeability has to be taken into account in the description for the relaxation time to get a reliable estimation of the interfacial tension. Our experiments show that for these systems the effective permeability is inversely proportional to the interfacial tension, gimel(eff) proportional to 1/gamma, and proportional to the square of the interfacial thickness, gimel(eff) proportional to xi(2). We find that the permeability is related to an effective diffusion coefficient as D-eff proportional to gimel gamma(eff). From this relation, we find that the diffusion coefficient is equal to 0.9 center dot 10(-9) m(2)/s, which is close to the self-diffusion coefficient of water; D-W(0) 2.3 center dot 10(-9) m(2)/s. This indicates that only water diffuses through the interface, and the diffusion coefficient is independent of the composition of the system for the concentration regime that is used.