Previous research has shown that multimodal interactions typical of autoparametric systems can be analytically modelled in simple structures. In this paper the vibration behaviour of two complex ultrasonic cutting systems is experimentally characterised. The energy exchange from the operating mode into a modal frequency close to half of the tuned frequency, and excitation of combination resonances consisting of two modal frequencies whose sum is close to the tuned frequency, are detected. The measured energy transfers from the primary responses to the internally excited modes are shown to be qualitatively similar to the theory and typical of autoparametric systems.