Fast-response differential-pressure transducers were used to continuously monitor the pressure difference between two nearby points along single gas slugs rising in long vertical tubes filled with liquid. Operating pressures in the 0.6 to 6.0-MPa range gave slugs of argon with densities in the 10 to 100-kg/m3 range, rising through mixtures of glycerol and water with viscosities in the 10−3- to 305 × 10−3-Pa·s. range. The transducer records suggest that, for all the liquids tested, there was flooding instability of the film flowing around the slugs when the gas density was raised sufficiently. With the assumption that flooding instability is due to the force on the waves moving along the gas–liquid interface, dimensional analysis shows that the observations with gas slugs correlate well with flooding data obtained in wetted wall tubes operating at atmospheric pressure. The work reveals strong limitations of current empirical correlations for the prediction of flooding in wetted wall tubes.