Churn flow is an important flow regime intermediate between slug flow and annular flow. A feature of this regime is the occurrence of very large waves travelling upwards over a liquid film substrate which may intermittently travel downwards. These waves are often formed close to the liquid inlet where their behaviour is usually difficult to observe. This paper describes a series of experiments using a test section with a specially constructed transparent liquid inlet. High-speed video recordings show clearly the process of wave formation and analysis of the recordings gave data on wave frequencies and typical velocities. Also, predictions of velocity and distance travelled by the waves were obtained via the application of a simple mathematical model that takes into account the forces acting on a circumferentially coherent wave.