The two-phase flow characteristics of gas in non-Newtonian fluids are important in many applications in various industries. Typical examples include the food processing industry, where it is sometimes desirable to aerate melted chocolate; also the chemical industry, where it is desirable to de-aerate non-drip paint. In the field of oil exploration the drilling mud is non-Newtonian. It is used for cooling the drilling bit, removing the drilled cuttings and maintaining the bottom hole pressure. Typical drilling muds have a yield stress to prevent sedimentation of drilled cuttings; they are also shear thinning to reduce pumping power and circulating pressures. An influx of gas into the wellbore is known as a gas kick and can lead to a blowout. One of the critical parameters in the evolution of a kick is the rate at which the gas influx rises up the well through the non-Netwonian drilling mud. The main parameters governing the gas velocity are the wellbore geometry (typically large annulus), the well inclination (vertical to horizontal), the volumetric flow rates, the volumetric gas fraction (0-100%) and the fluid rheology (yield stress and shear thinning). An experimental programme was set up at Schlumberger Cambridge Research to examine two-phase flows in large pipe and annular geometries using non-Newtonian liquids. The experiments were carried out in a 15 m inclinable flow loop, where air injection was used for the gas phase and aqueous xanthum gum solutions as well as water were used for the liquid. Using an instrumented test section it was possible to measure mean and spatially resolved void fractions together with gas and liquid velocities over a range of two-phase flow conditions. The results for vertical conditions showed that the air-water flows could be characterized as bubbly flows up to a void fraction of 15%, slug flow over 30% and transitional between. For the viscous muds the transition to slug flow occurred below a void fraction of 7.5%. This meant that for most flow conditions the gas would rise through the more viscous mud faster than it would in water. A description of the flow characteristics is developed in terms of the Zuber Findlay relationship for both pipe and annular flow geometries.