In the present work, the efficacy of the ultrasonic reactors for the degradation of p-nitrophenol, one of the most refractory compounds, has been investigated at pilot scale operation. An attempt has also been made to intensify the degradation process using different additives. The effect of air sparging, CCl4 addition, addition of solid particles such as TiO2 and CuO, and addition of ozone on the extent of degradation of p-nitrophenol has been studied. The sonochemical reactor used in the work is an ultrasonic longitudinal horn type reactor operating at 25 kHz frequency, power output of 1 kW, and capacity of 7 L. It has been observed that the extent of degradation increased for all the additives used in the present study possibly attributed to enhanced cavitational activity and generation of additional free radicals or oxidizing species. Maximum degradation of 93.5% was obtained using CCl4 as an additive at an optimum loading of 1 g/L. It appears that among different additives, additives giving additional free radicals and parallel oxidation mechanisms give more intensification as compared to additives which only enhance the cavitational activity in the system. It has been established that the use of additives at optimum concentration, which is dependent on the inherent cavitational activity in the reactor, results in significant intensification of the degradation process.