[en] Graphical abstract: - Highlights: • A simple sol–gel method for the synthesis of SnO₂–MgO nanoparticles is reported. • Band gap of SnO₂ can be tuned by varying the magnesium content in SnO₂–MgO. • SnO₂–MgO shows good photocatalytic activity towards degradation of methylene blue. - Abstract: SnO₂–MgO mixed metal oxide nanoparticles were prepared by a simple sol–gel method. The nanoparticles were characterized by power X-ray diffraction, scanning electron microscopy coupled with energy dispersive X-ray analysis, transmission electron microscopy and UV–vis diffuse reflectance spectroscopy. The XRD results indicate the formation of mixed metal oxide nanoparticles and also a decrease of SnO₂ crystallite size in the mixed metal oxide nanoparticles with increasing magnesium oxide content. The reflectance spectroscopy results show a blue shift of the band gap of SnO₂ in the mixed metal oxide nanoparticles. The photocatalytic activity of the SnO₂–MgO nanoparticles was tested using the photodegradation of aqueous methylene blue in the presence of sunlight. The results indicate that the mixed metal oxide nanoparticles possess higher efficiency for the photodegradation of methylene blue compared to pure SnO₂ nanoparticles