Multi-walled carbon nanotubes (CNTs) have been treated with gamma irradiation in air or by using dilute acids (H2SO4/HNO3) combined with 20 kHz ultrasound to compare their effects. The CNT microstructure has been investigated using transmission electron microscopy which revealed that both methods effectively modified the CNTs to overcome aggregation of the nanotubes, resulting in efficient dispersion in ethanol. The nature of the surface modifications was investigated using Raman and FTIR spectroscopies. The introduction of oxygenated species at the CNT surface was detected. At longer treatment times or high ultrasound intensities, the sonochemically assisted acid treatment showed the highest degree of reaction and functionalisation. Modification of the structure with gamma-radiation with doses of 100 kGy or 200 kGy also resulted in a reduction of defects, attributed to an annealing and reorganisation process. The observed effects could be correlated with the time and intensity of the ultrasound used or the dose and dose rate of the gamma-radiation. Both methods offer the possibility for processes with lower environmental inpact than those that currently exist. Our results also illustrate the importance of careful control over these experimental parameters if optimum results are to be obtained. (C) 2017 Elsevier B.V. All rights reserved.