Abstract <p>The results of analysis of the regularities of photodestruction of acid azo dyes in aqueous solution are presented. For this purpose, a fragment-based approach was employed, involving the decomposition of an organic molecule into molecular fragments consisting of chains of bonded atoms. The performed analysis made it possible to reveal the contribution of molecular fragments of substructures to the value of the logarithm of the first-order rate constant of dye photodestruction. An analysis of the contribution and direction of influence of fragments with different sequences of bonded atoms is presented. The kinetics of dye degradation in solution were compared with the lightfastness indices of dyes adsorbed onto a natural keratin substrate. A similarity was found between several individual multivalent fragments governing the photodestruction of acid azo dyes in solution and the lightfastness indices in the natural substrate.</p>