Moisture diffusion in a BMT resin has been investigated by submerging thin specimens in water. The weight gain of the neat resin deviates significantly from Fick's law in that the uptake continues to slowly increase over an extended time scale. A two-stage diffusion model can successfully fit the experimental data. This model considers the structural relaxation induced by absorbed moisture, which is a good plasticizer of the resin. Desorption and re-absorption experiments indicate that the structural relaxation is irreversible upon desorption of water molecules. Dynamic mechanical analysis results demonstrate the plasticizing and rejuvenating effect of water.