When sodium azide is irradiated with ultraviolet light the rate of photolysis at first decreases and then becomes constant. Two reaction mechanisms are involved: the first occurs only in salt which has not previously been irradiated, whereas the second requires the presence of metallic sodium. These two reactions are interpreted as the formation and linear growth of nuclei in the dislocation network of the salt and an equation developed from this model gives good agreement with the experimental results. Incorporation of Fe3+ ions in NaN3 leads to a greater number of nucleus-forming sites.