Factors limiting the activation of indium in silicon are examined via the ab initio pseudopotential technique. The role of carbon in the enhancement/retardation of activation/diffusion respectively is clarified. It is found that (1) adjacent substitutional indium atoms are deactivated. Only second neighbour sites of indium are activated unlike the case of boron, where all substitutional sites remain activated. (2) Silicon self-interstitials deactivate indium by trapping them on substitutional sites. Carbon, on the other hand, traps such self- interstitials with higher binding energy and prevents them from deactivating indium. (3) Since both indium and carbon diffusion is interstitial mediated, carbon reduces indium diffusion on account of its higher binding energy with the self-interstitial. Moreover, the release of the carbon interstitial is more favourable than the release of the indium interstitial from a carbon-indium pair. Therefore, carbon minimises indium interstitial diffusion. (4) Arsenic enhances de- activation of indium by neutralisation and by strong binding on adjacent substitutional sites. Furthermore since the release of the indium interstitial is more favourable in comparison to the release of the arsenic interstitial from the indium-arsenic pair, indium diffusion is enhanced in the presence of arsenic.