An experimental study of the liquid entry region during flooding is reported for a 50.8 mm i.d. vertical tube. The liquid entry device was a porous plastic tube into which film thickness measurement probes were mounted. Simultaneous measurements of the time-varying film thickness were made at locations below, inside and above the porous inlet. The weight of the evidence from cross-covariance analysis of these data does not appear to support models which assume upward propagation of waves above the feed point as the mechanism by which steady-state flooding takes place. Neither was there evidence that waves grow large enough to bridge the tube. Instead, the liquid film appears to undergo a transition from countercurrent to cocurrent flow inside the liquid feed. The pressure drop across the length of the porous tube was also measured and found to be significantly higher than that previously reported at locations above the liquid inlet. In part II [Int. J. Multiphase Flow 20, 235 247 (1994)] a film model based on these data is explored.