Precision Measurement of Half-Lives and Specific Activities ofU235andU238

New determinations of the half-lives of ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ have been made. Improved techniques have allowed the half-life values to be measured with greater accuracy than has been heretofore achieved. Samples were prepared by molecular plating and counted in a intermediate-geometry $\alpha$-proportional counter with an extremely flat pulse-height plateau. The small amount of residual nonplated uranium was counted in a $2\pi$ counter. Energy analysis with a silicon-junction detector was used to measure the presence of "foreign" activities. For ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$, the measured specific activity was (4798.1±3.3) (dis/min)/(mg ${}_{}{}^{235}{}_{}{}^{}\mathrm{U}$), corresponding to a half-life of (7.0381±0.0048) × ${10}^8$ yr. For ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, the specific activity was measured as (746.19±0.41) (dis/min)/(mg ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$), corresponding to a half-life of (4.4683±0.0024) × ${10}^9$ yr. Errors quoted are statistical (standard error of the mean), based upon the observed scatter of the data. This scatter exceeds that expected from counting statistics alone. We believe that systematic errors, if present, will no more than double the quoted errors.