Experiments on rotating channel flow, where both the primary (induced by a Coriolis instability) and the secondary instability are triggered independently, are described, focusing on the development of a secondary instability consisting of high-frequency travelling waves and their subsequent breakdown. Detailed hot-wire velocity measurements of the secondary disturbance are made and the phase speed and growth rate for various frequencies are determined. It is shown that the frequency of highest growth rate is close to that which is observed for naturally developing flow. Some information on the later stages in the transition process is obtained from frequency spectra, which show that interaction between various modes gives rise to stochastic low-frequency disturbances, which may play an important role in the transition process. A theoretical model of the disturbance structure is described which is used to explain some of the measured results and also allows the determination of the disturbance cross-stream flow field from only streamwise velocity measurements.