The purpose of the present study is to investigate the effects of liquid viscosity on the mean liquid film thicknesses, wave heights, and gas-liquid interfacial shear stresses in the vertical-upward cocurrent annular flow in a 26.0 mm inner diameter tube. Water and glycerol solutions were used as working fluids to change the kinematic viscosity of liquid from 0.85 x 10(-6) to 8.6 x 10(-6) m(2)/s. The mean liquid film thicknesses and wave heights were determined using the signals of time-varying cross-sectionally averaged holdup which were detected by a constant current method at a distance of about 3.5 m from an air-liquid mixer. The pressure gradients were also measured by a U-tube manometer. As a result we proposed correlations for the mean liquid film thicknesses and the interfacial friction factors. In addition a method to estimate the pressure drops is proposed and verified that the calculated values are in good agreement with the measured values.