Carbon-rich ＇carbonaceous＇ meteorites contain several types of dust grains with an isotopic signature that identifies them as being of pre-solar origin(1-3). Of these grains, diamonds are of particular interest : such grains are by far the most abundant, and they host an isotopically anomalous ＇Xe-H＇ component (characterized by a relative overabundance of the heaviest stable isotopes of xenon) which constitutes a notable fraction of the total amount of xenon in unprocessed ＇primitive＇ meteorites. The isotope abundance ratios of this Xe-H cannot be accounted for by the canonical processes responsible for nucleosynthesis of the elements heavier than iron, An ad hoc neutron-capture process has been postulated(4-6) to explain the observed isotope abundance ratios, but it has also been pointed out that standard ＇r-process＇ nucleosynthesis (in supernovae) could work if the stable isotopes were somehow separated from their radioactive precursors in the first few hours after the explosion(7). One way to distinguish between these mechanisms is to determine anomalies correlated for the heavy stable isotopes of tellurium in pre-solar diamond grains, Here we report such measurements, which support the suggestion that the isotopes were separated : the competing neutron-capture process cannot produce the observed abundances.