This study investigates the photocatalytic degradation of dyeing wastewater by iron-rich montmorillonite (MMt) under a 25W low-pressure mercury lamp (=254nm, I=48.4W/cm2). Exposing MMt to UV irradiation worked to catalyze the photobleaching of wastewater used in the dyeing process. COD removal efficiencies of 65.0% and 74.0% were achieved in 1g/L MMt suspensions at a pH of 3 in the absence and presence of 30min of UV irradiation, respectively. A decolorization efficiency of 71.4% was reached with 1g/L MMt suspensions at a pH of 3 after 30min of UV irradiation, while only 4.9% decolorization was attained in darkness. These results indicate that iron-rich MMt is a good photocatalyst for decolorization and COD removal in wastewater from dyeing processes. A low pH value is beneficial to the decolorization of dyeing wastewater. A preliminary photocatalytic mechanism is proposed: the iron in the MMt is responsible for the decolorization of the wastewater because it produces a highly oxidative species of hydroxyl radicals (center dot OH).