Nitrogen in silicon is known to affect dramatically the properties of voids. A plausible mechanism could be vacancy trapping by nitrogen interstitial species, mostly by the minor monomeric species (N1) with only a negligible contribution of the major dimeric species (N2). However, a more careful analysis of the published data shows that in Czochralski silicon no vacancy trapping occurs at the void formation stage (around 1100oC). The implication is that the trapping reaction, V + N1, although favoured thermodynamically, is of a negligible rate. Therefore, the nitrogen effect on voids in Czochralski Si is entirely due to nitrogen adsorption at the void surface. Quite a different mechanism operates in Float-Zoned crystals where voids are formed at lower T. Here vacancy trapping by N2 seems to be responsible for void suppression.