In age determinations of minerals in which thorium predominates, some inconsistencies have appeared when the value used for the disintegration constant of thorium is that adopted by the International Radium-Standard Commission (1930). This value differs from the original Geiger-Rutherford value by 30 percent. The present research was undertaken to determine the disintegration constant of thorium by the method of counting alpha-particles. Easily weighed sources were used. The material was obtained from a thorite from the same lot that was used by Geiger and Rutherford. The thorite contained only a small amount of uranium and correction for the ionium present was made. Various checks on the counting are given. The half-value period of thorium was found to be ${1.39\left(10\right)\mathrm{}}^{10}$ years and the branching ratio, thorium C' to thorium (C+C'), is 0.663. A method of impounding the thoron, yet permitting alpha-particles to escape, is described.

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