Experiments have been conducted on countercurrent flow of air and water in tubes of 25, 67 and 99 mm. diameter. Measurements of the pressure gradient, film. thickness and down flow rate were made for a range of air and water flow rates under pre- and post-flooding conditions. The data showed that the pressure gradient did not increase appreciably until just before the onset of flooding. The mean film thickness under pre-flooding conditions was also predicted well by the falling film correlations. However, both quantities showed significant deviations from this pattern under post-flooding conditions. Existing correlations for predicting the pressure drop under post-flooding conditions gave poor prediction of the present data as well of the data from the literature. New correlations are presented for the pressure gradient and the mean film thickness under post-flooding conditions.