Flooding during adiabatic two-phase countercurrent flow in a vertical and 60 degrees inclined (to the horizontal) 30 mm i.d. tube is investigated. Experiments are conducted with air, argon, helium, hydrogen, water, methanol, isopropanol and aqueous methanol solutions. An improved flooding correlation is proposed which accurately correlates the data. In the past, a number of proposed dimensionless groups often only succeeded in correlating their own data. The present correlation excellently correlates the data of three previous investigations. The findings clearly show that some flooding data had been misinterpreted in the past. The fluid densities have the strongest effect on the flooding gas velocity. The effect of the liquid viscosity is relatively stronger than that of the surface tension. Flooding is independent of the gas viscosity.