Polynuclear aromatic hydrocarbon (PAH) contamination has spread throughout the globe with background levels now found in virtually all sections of the ecosystem and environment. The mutagenic and/or carcinogenic properties attributed to many of these compounds, and frequency of occurrence and concentration in the environment, has driven research into safe methods of removing contamination, whilst avoiding the use of harmful solvents or the formation of even more hazardous compounds. Ultrasound is currently used in industry and research to propagate and accelerate chemical reactions, opening reaction pathways which otherwise would not be observed. In the study of the degradation of PAHs through ultrasonic irradiation, the breakdown of an aqueous solution of phenanthrene in a sonochemical reactor utilising a 30 kHz probe system, operating in batch mode, has been investigated. The phenanthrene molecule was studied and used as a model PAH molecule. It was chosen due to the structural similarities to many of the higher order PAHs currently recognised as being hazardous to health. The influence of several parameters on the degradation of phenanthrene are reported (power ultrasound energy, temperature and light). Qualitative analysis using HPLC and quantitative analysis using UV/Vis photo-spectrometry confirmed that a 88% reduction in the peak observed phenanthrene concentration was achieved over 240 min of sonocation. Whilst there was the potential for the formation of recalcitration and rearrangement products, no higher order PAHs were observed and a 80% reduction in total monitored UV fluorescence and hence, aromaticity/conjugation, was observed.