Quantum dots of Cd(Se,S) and fluorescent magnetic nanocomposites (Cd(Se,S)-magnetite) were used as photocatalytic agents in the photodegradation of methylene blue (MB) under UV irradiation at pH 6.5. Quantum dots and magnetic nanocomposites were characterized by X-ray diffraction (XRD), UV-Vis, photoluminescence and Fourier Transform Infrared (FT-IR) spectroscopy. The photo-induced degradation of MB was monitored using High Performance Liquid Chromatography (HPLC) at 660 nm and titanium dioxide (anatase and aeroxide P25 forms) was used as the photocatalyst standard. A degradation of 99.1% and 90.0% of MB was achieved in the presence of 160 mg L^-1-quantum dots and the magnetic nanocomposite, respectively, after 4.5 hours of UV-irradiation. Instead, 45.9% and 100% of MB degradation was achieved using 160 mg L ^-1 of TiO<inf>2</inf> anatase and aeroxide P25, respectively. The degradation products were studied by mass spectrometry (MS) and the results evidenced the formation of azure B, A, C and phenothiazine. The reuse of the magnetic nanocomposites (i.e., after one photo-degradation cycle) allowed a maximum photo-degradation capacity of 65%. The results suggested that the nanocomposite has 10% less photodegradation capacity than the widely used catalytic agents such as TiO<inf>2</inf> aeroxide P25. © 2013 The Royal Society of Chemistry.