This paper deals with the prediction of flooding in wetted-wall flow along vertical surfaces. A critical review of some published data, on flooding in wetted-wall tubes, is initially presented, and the data are compared with the predictions of two well-known empirical correlations. An alternative method is then developed for the prediction of flooding, based on physical reasoning coupled with dimensional analysis. The main conclusion is that, for any given set of properties of the liquid [in fact, for a constant value of the parameter Z = sigma (rho/g mu(4))(1/3)], the dimensionless interfacial shear stress between gas and liquid, at the point where flooding is about to start, should correlate with the Reynolds number for flow in the liquid film, Re = 4Q/v. Published data are then used to establish the form of that correlation, for systems where the liquid is water at room temperature (i.e; Z congruent to 3360). The proposed correlation is given as eqs (14) and (15) in the paper and these equations are then manipulated to yield a simple approximate criterion for the prediction of flooding, in vertical round tubes; according to this new criterion, flooding may be expected to occur whenU-g + u(i) > 0.353 [(rho g sigma)D-2/mu(g)rho(g)(3)](1/7) Fwhere u(i) and F are known functions of Re.This criterion is then shown to accurately predict the recent finding that gas slugs will be subject to flooding at high pressure.