The energies of the alpha-particles arising from the reactions ${\mathrm{Li}}^7(p,\alpha ){\mathrm{He}}^4$ and ${\mathrm{Li}}^6(d,\alpha ){\mathrm{He}}^4$ have been measured by stopping the particles in a calibrated, variable pressure, air-filled absorption cell. The use of collodion windows on the cell instead of mica, the use of thin instead of thick targets, and the prevention of the formation of oil films on the targets, were improvements in technique over the older determinations of Oliphant, Kempton, and Rutherford. The energy release in the ${\mathrm{Li}}^7$ disintegration was found to be 17.28±0.03 Mev; that for the ${\mathrm{Li}}^6$, 22.20±0.04 Mev. These new values are used, together with the energies of the beryllium-proton disintegrations measured with the electrostatic analyzer, to set up masses for the light atoms concerned. The following values are obtained: ${\mathrm{Li}}^6$, 6.01682±0.00011, ${\mathrm{Li}}^7$, 7.01784±0.00009, ${\mathrm{Be}}^8$, 8.00766±0.00015, ${\mathrm{Be}}^9$, 9.01486±0.00013.

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