[en] Field and laboratory measurements were made to test the validity of methods for calculating radon-flux attenuation through earthen covers as described in A Handbook for the Determination of Radon Attenuation Through Cover Materials, NUREG/CR-3533. Radon flux was measured at several sites on two, different, earthen-cover systems over uranium-mill tailings. Afterwards, the underlying materials were removed; and their properties which govern radon diffusion were measured. The validity of the diffusion equations presented in the handbook was established by the generally good agreement between the measured radon flux and the flux predicted when measured values of soil properties were used in these equations. Also, approximate values presented in the handbook for various soil properties were compared with those measured on samples collected at the test sites and were generally found to agree within a factor of four. When these approximate values were used in the diffusion equations, the predicted fluxes were larger than the measured values by factors of up to 31. This implies that the diffusion equations will overestimate the depth of cover soil required to attenuate radon flux to a prescribed level if these approximate values are used. However, investigation of the theoretical relationship between the radon flux from an earth-covered tailings pile and the thickness of that cover indicated that the latter would only be overestimated by a factor of up to 1.5 at field sites similar to those examined in this study. In addition, measurements of radon flux over the drill holes indicated that these cover defects, penetrating the entire thickness of the cover but accounting for only 0.07% of its surface area, had negligible effect on the average flux from the cover system. 13 references, 1 figure, 12 tables