The influence of deviations from TaSi2stoichiometry in cosputtered, amorphous Ta–Si films on interface reactions in contacts to silicon at 900 °C was investigated by TEM of thin cross sections through the contact windows. For Si rich films, the excess Si precipitates epitaxially in the contacts, whereas pits are formed for Ta rich films. Since the lateral Si transport involved ranges over several tens of microns, even a very small deviation from TaSi2stoichiometry leads to strong degradation of the contacts. The interfacial oxide has no influence on the contact reactions. Since slight deviations from disilicide stoichiometry cannot be avoided with cosputtering, the realization of a low‐resistive, high‐temperature‐stable TaSi2interconnection system with good contacts to Si would additionally require a high‐temperature‐resistant barrier layer. Experiments with a crystalline TaSi2pad and a TaN0.8barrier layer show insufficient stability at 900 °C, but suggest that most of the large lateral Si transport happens during the initial stage of crystallization.