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Interferometric fiber-optic gyroscopes exhibit time-dependent rate error patterns during operation due to environmental stress on the fiber coil. Short-term errors, equilibrating on the order of minutes to several hours, are attributed to nonreciprocity due to thermal gradients. Long-term rate errors, equilibrating on the order of days to weeks, have not been thoroughly addressed. In this study, we show that diffusion of moisture into or out of a sense coil can cause long-term rate errors. To calculate this effect, we measured the effect of moisture on the mechanical properties of the optical fiber coating. Using these data, we modeled diffusion in a sense coil with finite-element analysis. The rate error is calculated with an integral that is similar to that used by Shupe and others. A variation in water concentration in the coil due to diffusion causes changes in the properties of the fiber coating. This in turn produces nonreciprocal stresses on the waveguide and leads to a rate error.