This study focuses on the low-pressure spraying of viscous liquids (mu = 60, 147 and 308 mPa s) using four types of internal-mixing twin-fluid atomizers. We compare two well-known designs, namely the Y-jet and "outside in gas" (OIG) effervescent atomizers, with our new design (CFT) and an "outside in liquid" (OIL) configuration for the effervescent atomizer. The atomizers were operated by two gas inlet pressures (0.14 and 0.28 MPa) and various gas-to-liquid ratios (GLR = 2.5%, 5%, 10% and 20%). The comparison focused on internal liquid-gas flow, spray stability, primary breakup, and droplet size. The primary breakup was investigated using a high-speed camera. A near-nozzle spray pattern was related to the ratio of forces, which affects liquid deformation, by dimensionless numbers. The breakup was driven mainly by air resistance in the OIG, OIL, and CFT atomizers and by surface tension in the Y-jet atomizer. The OIL and Y-jet atomizers provided the most stable spray, regardless of the working regime or atomized liquid. The OIL atomizer produced the smallest droplets at low GLRs, while the droplet sizes for the Y-jet atomizer increased significantly at low GLRs. For the OIG atomizer, spray stability was influenced by the GLR, with the best stability being achieved at a GLR of 10% and 20%. The presence of large droplets at a low GLR caused an increase in droplet size. Switching the inlet ports of the effervescent atomizer (OIG-OIL) affected the internal flow, which differed under the same working regimes for these two configurations. The internal flow pattern of the OIL atomizer was estimated to be annular for all regimes, while for the OIG atomizer, it changed from a plug to slug flow with an increase in the GLR. (C) 2015 Published by Elsevier B.V.