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This paper deals with experimental investigations on syntactic foam and its behavior at outdoor conditions. For the experiments, plate specimens of syntactic foam of a thickness of 3mm are used. Hollow glass microspheres with untreated and silane-coated surface as well as microspheres with a low alkali ion concentration are applied as filler. During a test period of 50 days the specimens are stored in deionized water, which is tempered to 50°C to accelerate the water absorption. The weight of every specimen is measured during the water storage to determine the increase of weight due to water ingress. The dielectric permittivity and the loss factor are measured in constant intervals during the whole test period. As a result of these investigations, a different behavior of the syntactic foam filled with different hollow microspheres is observed. The syntactic foam specimens with untreated hollow microspheres show a significant increase of their permittivity and loss factor during the test period, whereas the dielectric properties of the specimens with silane-coated microspheres and microspheres with low alkali ion concentration are not influenced by water absorption. The results show that syntactic foam can potentially be used as insulation material for outdoor applications if suitable microspheres are used as filler.