A method for the surfactant-free coating of gold nanoparticles with thiols using sonochemistry is presented. The gold nanoparticles were prepared by a modified Zsigmondy method, affording good con- trol over the particle-size distribution, and the thiol coating was performed by the sonication of a bipha- sic system consisting of a nanoparticle suspension in water and thiols in toluene. The effects of two important reaction parameters on the particle morphology, viz. sonication time and thiol concentration, were investigated in detail using transmission electron microscopy. The effect of the thiol chain length was also studied. We show that the morphology of the coated particles is determined through a compe- tition between two opposing effects: particle fusion, due to the sonication conditions, and digestive rip- ening, due to the action of the thiols. Additionally, we illustrate the utility of our technique for various applications, including surface-enhanced Raman scattering from bound molecules, and further function- alization using a thiol-exchange reaction. Our technique paves the way for an efficient synthesis of thiol- coated AuNPs of different shapes and sizes, suitable for a range of diverse applications.