A study was made of the effect of moisture on the emanation coefficient and radon flux from uranium mill tailings. A sharp rise in emanation coefficient occurred as the moisture content was increased from the absolutely dry state to 2% water by weight. The emanation coefficients from water-saturated tailings were about four times those from absolutely dry materials. Radon flux was measured from columns of dry, moist and water-saturated tailings. The highest flux came from the column filled with moist tailings. This can be explained by the effect of moisture content on the emanation coefficient. Water-saturated tailings gave the lowest flux because of the much lower diffusion coefficient of radon through water.