The sonochemical degradation of alachlor has been investigated using ultrasonic horn. Initially the effect of operating initial pH has been investigated over the range of 2-11 followed by the effect of different intensifying additives such as hydrogen peroxide and carbon tetrachloride. It has been observed that acidic conditions favor degradation and also both the additives result in enhanced extent of degradation. The combination of ultrasonic horn with conventional Fenton and advanced Fenton process was also investigated. With an objective of deciphering the controlling mechanism for the sonochemical degradation, the effect of addition of hydroxyl radical scavengers such as sodium bicarbonate and n-butanol has been studied and it has been observed that the degradation of alachlor is strongly inhibited in the presence of sodium bicarbonate and n-butanol due to the free radical scavenging effect. Some preliminary experiments have been performed at larger scales of operation to check the efficacy of sonochemical reactors for large scale application. Kinetic studies revealed that all the systems under investigation followed first order degradation except US/Fenton and US/advanced Fenton where second order kinetics was observed. Analysis of the intermediate products of the alachlor degradation process has also been made using LC-MS. (C) 2012 Elsevier B.V. All rights reserved.