The complex chemical and physical processes occurring in hydrating cement pastes are not fully understood. The properties of cement setting and the influence of different additives are encompassed by the delayed nucleation and the protective layer theories. Each of these two theories is supported by several different experimental observations. In order to resolve this question further, the proton nuclear magnetic resonance (NMR) data of the hydrating cement pastes are analysed and discussed. This analysis shows that NMR data favour the osmotic model of cement setting.