Experimental studies on atomization in an internal-mixing twin-fluid atomizer are reported over a wide range of liquid viscosity, gas supply pressure and Gas to Liquid mass Ratio (GLR). Among all test conditions, the finest sprays were obtained at an axial distance of 150 mm and Sauter Mean Diameter (SMD) was dominated by GLR while being nearly independent of viscosity and pressure. However, droplet size distributions changed obviously when viscosity increased to 120 mPa s. More large droplets were produced and large droplets always took up a large proportion, and spray quality deteriorated, though SMD changed little. At large axial distances (250 or 400 mm) SMD increased with the increase of viscosity when the atomizer was operated at high pressure and GLR. The decay of axial velocities along the spray centerline was stronger at a larger viscosity. In the radial direction, an increase in viscosity made distributions of both SMD and velocity flatter. An increase in GLR generated more small droplets in the expense of large droplets and hence improved spray quality. Pressure had a weaker promotion on the droplet size distribution when comparing to GLR. (C) 2012 Elsevier Ltd. All rights reserved.