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The term Sheet Molding Compound (SMC) is applied to composites formulated with unsaturated polyester resin (UPE), glass fibers as reinforcement and other additives: thickening agent, low profile agent, inorganic filler (CaCO3), etc. The understanding of the hydrothermal ageing effects of this material is an important factor for its improvement, because this heterogeneous material very easily deteriorates in the presence of humidity. The effect of this kind of ageing is analysed using dynamic mechanical thermal analysis. Our mechanical spectrometry results, show that the hydrothermal ageing produces two types of effects in our model class A SMC samples: firstly, some partially reversible physical effects associated, on the one hand, with the hydrophilisation of the UPE network and, on the other, with the thickening phenomenon (formation of coordinative bonds between the carboxyl groups of the ester linkages and the unsaturated Mg ions). Secondly, some non-reversible chemical modifications due to the partial hydrolysis of the low profile agent (a small amount of PVAc is transformed to PVA and to acetic acid).