This paper describes an experimental study on the onset of water accumulation above a perforated plate in a vertical air-water countercurrent flow. We experimentally investigate the effects of holes with the relatively large diameter (5 cm), the number of holes (4 holes and 12 holes), the thickness of plate (1 cm and 4 cm), asymmetric water injection, and the location of the air vent line on the onset of water accumulation. The present results indicate that the onset of water accumulation is promoted by a smaller diameter of hole, a smaller number of holes, and symmetric water injection whereas it has negligible dependence on the location of air vent line in the upper plenum. It turns out that the nondimensional superficial velocity, j(k)(*), fits the data better than either the Kutateladze number, K-k, or the interpolation parameter, H-k(*). We develop a Wallis-type correlation for the onset of water accumulation when the diameter of the holes ranges between 0.7 and 5 cm, and the number of holes is greater than six. (C) 2006 Elsevier B.V. All rights reserved.