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• Physical ageing of polyurea with different stoichiometric ratios was investigated. • Fick's model well fitted the water absorption kinetics of polyurea. • Diffusion sequence of water molecules in polyurea was determined. • Compact hard domains were rarely affected by water molecules. • Mechanical properties decreased significantly following water absorption. The evolution of molecular dynamics and microstructures of polyurea (PUA) elastomers with different stoichiometric ratios upon water absorption process was investigated. Water absorption kinetic of PUA was studied using Fick's model, and water molecules interacted in the following sequence: free C O > monodentate hydrogen-bonding C O > bidentate hydrogen-bonding C O. The utilization of dielectric relaxation spectroscopy to explore the molecular motions of PUA elastomers before and after water absorption revealed the impact of water absorption on both the molecular dynamics of soft segments as well as the interface between soft and hard domains. 2D Fourier transform infrared spectroscopy results showed that water molecules interacted with the soft domains, followed by the loosely-packed hard domains, and finally with the compact hard domains, resulting in the partial disintegration of hard domains evidenced by small-angle X-ray scattering. The tensile strength of the studied PUA elastomers decreased by half after water absorption, which was attributed to the plasticization effects induced by the absorbed H 2 O molecules, as well as disassembly of the loosely-packed hard domains and the interface regions. [ABSTRACT FROM AUTHOR]