Abstract. The internal conversion process of the 165.8 keV 7-ray transition in the decay of 139Ce has been reinvestigated. The emission rate of the K-shell internal conversion electrons was determined by an (electron)-(X-ray)-(X-ray) triple coincidence experiment using a magnetic/?-spectrometer and two Si(Li) detectors. From electron spectra recorded as a function of momentum conversion electron ratios were also determined. The disintegration rate of the sources has been taken from 4 rc/?-7 coincidence experiments. The following results were obtained: the probability for internal conversion in the K-shell tcK=0.1719