Spalling, which is a phenomenon encountered when high-performance concrete is exposed to high temperature, can lead to large economical damage and can be a major safety hazard. Moisture distribution in concrete during exposure to fire is of paramount importance for understanding the complex mechanism of this phenomenon. To capture in its fullness this mechanism, it is crucial to account for the heterogeneous nature of concrete. In this paper, the first 3D analysis of moisture distribution in concrete at high temperature through in-situ neutron tomography is presented. The world-leading flux at the Institute Laue Langevin in Grenoble, France allowed capturing one 3D scan per minute, which is sufficient to follow the fast dehydration process. The paper describes the experimental setup with the heating system and discusses in detail the framework of the neutron tomography test. Quantitative analysis showing the effect of the aggregate size on the moisture distribution is presented.