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Isocyanate groups present in the precure mix of polyurethane-based composites are highly moisture sensitive. Binding of water with isocyanate groups occurs at the expense of cross-linking in the cure process. Consequently, the dielectric properties of the resulting material may alter due to i) the presence of bound water in the material and ii) changes in the dielectric properties of the polymer due to reduced cross-linking. In this work, polyurethane-based composite samples were cured in various controlled relative-humidity environments, created using saturated salt solutions. The dielectric constant of the materials was measured as a function of cure time, in the frequency range of 0.5 to 18 GHz. In this way the relationship between relative humidity during cure, and dielectric properties of the resulting composites, is demonstrated.