In the present paper, experimental results of the investigations of the effect of liquid viscosity on the phase distribution in two-phase air-liquid slug flow in horizontal pipes are represented. As the liquid phases, oil with viscosities in the range from 14 to 37 mPas and water are used. The experimental investigations are conducted in a horizontal pipe with an inner diameter of 59 mm. The superficial liquid viscosities and the superficial air velocities in the test section are varied in a range from 0.15 to 1.5 mis and from 0.1 to 13.5 mis, respectively. The liquid hold-up within the liquid slug zone, the liquid holdup in the elongated bubble zone and the flow averaged liquid holdup are measured by means of a multibeam gamma-ray densitometer. The results indicate that there are significant differences in the distribution of liquid and gas when oil and air or water and air are used in the experiments. By increasing the liquid viscosity, increasing values for the Row averaged liquid holdup and the volume averaged liquid holdup inside the liquid slug and the elongated bubble zone are obtained.