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