The aim of the present study is to decolorize azo dyes containing (-N=N-) bonds such as Reactive Black 5 (RB 5), Basic Blue 3 (BB-3) and Disperse Blue 373 (DB 373) in the presence of ultrasound (US) and rare earth (RE), La+3, Ce+3, Pr+3 and Gd+3 ions in aqueous solutions. In the presence of US (20 kHz, 250 W) alone, these dyes, in the concentration range 8-144 ppm, were decolourized to different extents, RB5 (similar to 20-17%); BB3 (similar to 40-21%) and DB373 (similar to 47-35%) at different time intervals. However, in combination US with REs, the removal was greater than 90%. The degradation followed pseudo-second-order kinetics.