An experimental investigation of gas-liquid flow in horizontal pipes is conducted. Consideration of a stationary liquid phase allows the direct calculation of gas wall and interfacial shear stress from measurements of gas velocity, pressure drop, and liquid height without the need for any empirical input. The experiments are conducted to determine the effects of liquid viscosity. The results show that commonly used correlations for interfacial friction factor, which are generally developed in rectangular ducts, are inadequate, especially under conditions of increased liquid viscosity. It is shown that more success can be achieved if the effect of liquid viscosity is considered in the transition to wavy flow. New correlations for shear generated by smooth, rippled and wavy interfaces are proposed.