There is current interest in using high-intensity ultrasound to perform a range of chemical transformations, including polymerisation reactions. In this work, the technique of radical trapping, primarily using DPPH, has been used to measure radical production rates in a range of monomer and related systems when exposed to high intensity ultrasound. It has been shown that realistic rates of production can be obtained around room temperature equivalent to thermal decomposition rates > 100degreesC, making sonication a viable method for initiating polymerisation. Rates of initiation in a two-phase organic in water system have also been measured. Some of the initiating species have been identified recording the ESR spectra of adducts with spin traps, although further analysis is needed before the complete range of radicals produced can be identified.