The two-phase pressure drop in vertical industrial pipes is mainly determined by gravitation and acceleration of the fluid, which means that the void fraction is key an important parameter in any model to predict pressure drops. Typically, these models are applied in industry to size pumps and, e.g., emergency relief systems. There is a shortage of void fraction data in the literature for liquids with a dynamic viscosity above 1000 mPa s. Adiabatic experiments have been performed of mixtures of nitrogen and solutions of polyvinylpyrro lido lie (Luviskol((R))) in water with dynamic viscosities in the range 900-7000 mPa s. Inner tube diameter was 54.5 mm. Mass flux and quality were varied in a wide range: 8-3500 kg/m(2)/s and 0-82%, respectively. The corresponding superficial velocities were 0.005-3.4 m/s for the liquid and 0-30 m/s for the nitrogen. For comparison, reference measurements were taken of mixtures of nitrogen with water (I mPa s). Care has been taken to measure only well-developed flows. Time-averaged local void fraction profiles have been determined with a linearly traversed gamma-ray densitometer. Analysis shows that at high superficial gas velocity (gas Reynolds numbers in the range 0-1.2 x 10(5) have been studied, liquid Reynolds numbers in the range 0.2-1.7 x 10(5)) the total superficial velocity profile is peaking in the centre of the tube. With increasing superficial gas velocity the peaking gets stronger. It is shown that time- and space-averaged void fractions are not well predicted with existing correlations. Two new correlations are presented, one of them in terms of the distribution parameter. The other, in terms of the velocity slip, unifies the results of low- and highviscosity mixtures. (c) 2007 Elsevier Ltd. All rights reserved.