When 4,4′-dinitrostilbene-2,2′-disulfonate (DSD) is submitted to UV or visible light in aqueous solution, the main photochemical process is a reversible trans–cis isomerisation, the cis form absorbs at shorter wavelength than the trans isomer. The UV spectrum of cis form may be obtained not only by Fischer’s method but also by using HPLC with UV detection at isosbestic point (311nm). Quantum yields were evaluated at 0.30 and 0.24 for trans→cis and cis→trans isomerisation, respectively.Photolysis also occurs but only with a very low quantum yield [(2.2−2.4)×10−4]. For this reason photocatalysis may be useful to accelerate the elimination of DSD from waste waters. Photocatalytic transformation is more efficient with TiO2 Degussa P25 than with TiO2 Millennium PC50 in spite of pure anatase composition of the latter. Immobilization of TiO2 on pumice stone is an interesting method to eliminate the problem of filtration implied by the use of slurries. To evaluate disappearance by UV detection it is recommended to detect at isosbestic point to do not interfere with isomerisation.