Abstract We have studied interactions of chlorine under low pressure with tantalum at high temperatures in an ultra high vacuum apparatus where all desorption products, including unstable and condensible species are analyzed by mass spectrometry. At low chlorine coverage, atomization, characterized by a zero coverage reactive probability of 1 is the only observable process. When the coverage increases, desorption of atomic chlorine is progressively replaced by desorption of tantalum tetrachloride TaCl 4. All the experimental results are consistent with a model, previously proposed for the oxygen-transition metals system. This model is based on the following elementary steps: (a) dissociative adsorption of chlorine; (b) desorption of atomic chlorine; (c) desorption of tetrachloride, only formed by surface reaction.