The catalytic degradation of Reactive Blue 4 (RB4) by an emerging ultrasound-assisted photo-Fenton process with artificial ultraviolet light was investigated. The photocatalytic degradation efficiency was determined by the decrease in Total Organic Carbon (TOC) content. The influences of pH and initial concentrations of Fe(II) and H2O2 on the RB4 solution mineralization were studied. Under the optimal conditions, TOC removal increased up to 94% in 60 min, and this system permitted the use of a low ferrous concentration of only 5 mg L-1. The RB4-mineralization process in the sono-photo-Fenton system can be described by a mechanism involving mineralization by direct photolysis, ultrasonically generated oxidative species, direct oxidation reaction with H2O2, radical reaction (mainly HO center dot) and thermal pyrolysis inside the bubble. The radical reaction in the bulk solution or in the vicinity of the bubble was found to be the main mineralization pathway. The contribution of different mechanisms to overall mineralization was the following: radical reaction (93.60%), direct reaction with H2O2 (2.56%), photolysis (1.92%) and reaction by ultrasonically generated oxidative species (1.92%). The influence of the Fe catalyst on the radical reaction was evaluated by conducting the reaction in the presence and absence of the scavenging agent potassium iodide (KI). The toxicity profile of sono-photo-Fenton process was also evaluated. (C) 2014 Elsevier B.V. All rights reserved.