An existing design of conductivity probe for the measurement of void fraction has been developed and tested. Calibration checks showed that the time averaged output of the instrument was close to linear over the whole range of void fraction, with a tendency to overestimate the true value. Computer modelling of the probe’s response to different void distributions supported the calibration results. Two meters were used to examine the dynamic Variation of void fraction in air-water flows in a vertical 32 mm diameter tube. Flow patterns ranging from bubbly to annular were observed. Examination of the void fraction traces and their probability distribution functions enabled the identification of six flow regimes : discrete bubble flow, spherical cap bubble flow, stable slug flow, unstable slug flow, churn flow and annular flow. The locations of flow regime transition boundaries on a flow pattern map, deduced from this objective analysis, are in agreement with the data of other workers. The unstable slug flow regime corresponds to a region some workers have identified as churn flow, it provides explanations for some of the apparent anomalies concerning the location of the slug to churn flow boundary. Cross-correlation of the signals from the two probes has provided statistical data on the lengths of slugs and Taylor bubbles which suggested modifications to the previous theories enabling the entry to and exit from the unstable slug flow regime to be predicted.