Physicochemical adsorption and degradation of the azo dye, methylene blue (MB), when irradiated by visible and UV light in aqueous ZnO suspension have been investigated. The novelty of this work is to investigate the effect of dye concentration up to 200 mg/l, keeping the nanocatalysts’ concentration invariant as 1 g/l. The nanocatalysts before and after degradation process have been analysed to understand the mechanism of dye removal using X-ray diffractometer (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal gravimetric analyser(TGA). The results show that UV light degrades the dye to its intermediates through chemi-adsorption, whereas the visible light induces physisorption of dye molecules on ZnO nanocatalysts. The XRD pattern of the ZnO nanocatalysts revealed no changes in the internal structure of ZnO after the degradation process, confirming thereuse of catalysts.