 tau forms aggregates in the brains of Alzheimer patients. Here, the authors identify conditions where the micro-dual binding repeats of tau undergo a phosphorylation-dependent liquid-liquid phase separation, leading to molecular crowding in the formed tau-liquid droplets and characterize them by NMR and other biophysical methods. This article was authored by Sismitha Ambedipidi, Jocsek Piyanat, Dietmar Riedl, and others.