Cesium isotopes produced by ${\mathrm{I}}^{127}(\alpha , xn)$ type reactions in the 60-inch and 184-inch cyclotrons were studied in Geiger counters, a mass spectrometer, beta-ray spectrometers, a sodium iodide crystal scintillation spectrometer, and a gamma-gamma coincidence spectrometer. The mass assignment of 30-minute ${\mathrm{Cs}}^{130}$ was verified. The 6.25-hour ${\mathrm{Cs}}^{127}$ was shown to decay predominantly by electron capture but also by the emission of positrons of 1.063-Mev and 0.685-Mev energy. Prominent gamma rays of 125 and 406 kev are observed. ${\mathrm{Cs}}^{127}$ exhibits branching decay to 75-second ${\mathrm{Xe}}^{127m}$ in about one disintegration in ${10}^4$. This isomer was isolated and studied in a scintillation spectrometer. $E3\mathrm{}$ radiation of 175 kev connecting $h_{\frac{11}{2}}$ and $d_{\frac{5}{2}}$ levels is followed by 125-kev $M1\mathrm{}$ radiation corresponding to a $d_{\frac{5}{2}}-d_{\frac{3}{2}}$ transition to the ground state. Forty-five minute ${\mathrm{Cs}}^{125}$ is shown to decay predominantly by electron capture and also by the emission of positrons of 2.05-Mev energy. Other lower energy positron groups are present. A prominent gamma ray of 112 kev is observed. In one disintegration in about ${10}^3$, ${\mathrm{Cs}}^{125}$ decays to a 55-second ${\mathrm{Xe}}^{125m}$. This isomer emits gamma rays of 75 and 110 kev which are believed to be $E3\mathrm{}$ $h_{\frac{11}{2}}-d_{\frac{5}{2}}$ and $M1\mathrm{}$ $d_{\frac{5}{2}}-d_{\frac{3}{2}}$ transitions, respectively. An incomplete study of ${\mathrm{Cs}}^{123}$ shows it to be a 6-minute positron emitter.

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