How stable are geranyl and farnesyl halides in water? Given the current increased interest in mimicking terpene synthases, we address this fundamental question by investigating their behaviour with regard to their ability to undergo hydrolysis and cyclisation reactions in the absence of a catalyst or chemical promotor. Some linear terpenoids with leaving groups, such as bromide, appear to have an intrinsic tendency to cyclise. In particular, hydrolysis or cyclisation of the respective water-insoluble bromide analogues by ultrasonic treatment was found to be effective. This technique ensured efficient dispersion of the hydrophobic oil droplets in the aqueous medium. It was found that even with short ultrasonic treatment times, the (2,3-Z)-configured terpenes neryl and Z-farnesyl bromide showed a pronounced tendency to cyclise, which in some cases led to high conversion and high selectivity. In contrast, omitting ultrasonic treatment resulted in a heterogeneous reaction environment, leading to a broader and more complex product spectrum, including the formation of dimeric ethers.