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The moisture sorption characteristics of polymer concrete and its components (polyester resin, unfilled and filled with diabase flour) on long-term (up to 15-year) exposure to water at different temperatures are studied. It is established that, during the long-term sorption and subsequent desorption at 20°C, the ratio of diffusion coefficients of the polymer concrete and the corresponding resin is equal to the value of time-moisture reduction function, which characterizes changes in the creep compliance of the materials. The evaluation of the diffusion coefficient of the composite from the properties of its components, by using various known heat-and-mass-transfer models, shows that most acceptable is the Kerner model. With account of volume content of pores, an estimate for the limiting moisture content in the composite is proposed. An analysis of sorption curves of the composite and the corresponding resin reveals that Fick's law does not describe the experimental results in the range of large times and/or elevated temperatures. In the case of polyester resin filled with diabase flour, the “composite effect” is expressed in a linear increase in the specimen mass (the rate of the increase is temperature-dependent. In the case of polymer concrete, the “composite effect” is expressed in mass losses, which can be described by Fick's law with a diffusion coefficient and a limiting moisture content both depending on temperature.