The effects of H2O contamination on lattices of natural uranium metal in D2O were measured in the exponential facility of the Savannah River Laboratory. The buckling changes associated with H2O contamination were determined for two lattices with moderator-to-fuel volume ratios of 12.3 and 14.6 over a range of H2O concentrations from 0.2 to 8.2 mol %. The agreement between calculated and experimental changes in buckling for these lattices was within ±25 × 10−6 cm−2. Similar measurements on seven other lattices with moderator-to-fuel ratios in the range from 31 to 212 were made for a change in the H2O concentration from 0.18 to 3.92 mol %. For these measurements the experimental change in buckling was about 15% greater than the calculated change.