An insight in mixing and in mass transfer processes in gas-liquid flows requires a reliable knowledge of the local gas holdup. Conventional methods for measuring the local gas holdup have shortcomings, such as costly measurement techniques, extensive signal processing, and limitations to highly turbulent flows. In this work, a simple and reliable method for measuring the local gas holdup in turbulent and highly turbulent churn flows is described. It was used to determine the local gas holdup in different locations inside an impinging-stream reactor aerated from the bottom.