In this work, we have studied the relative stability of perfect (PDLs) and faulted (FDLs) dislocation loops formed during annealing of preamorphised silicon. In particular, we have investigated the effect of the annealing ambient (N2, O2) on their thermal evolution. Transmission Electron Microscopy analysis shows that after short annealing times FDLs are the more stable defects if the ripening process is conservative or if the surface acts as a sink for Si interstitial atoms. On the contrary, when the surface injects Si atoms (oxidising conditions) or for long annealing times under conservative conditions, PDLs are stabilised. We have calculated the formation energies of these defects as a function of their sizes and we have found that a stability inversion occurs for sizes of about 80 nm. All the experimental results presented here are explained by making use of these calculations.