Airblast atomization of viscous liquids is carried out using a twin-fluid jet atomizer. As the atomizing air swirls downstream along the liquid jet, waves form on the surface of the liquid jet. Thus, the liquid jet sheds ligaments which subsequently collapse into small drops. The drop sizes and size distribution are measured using the laser diffraction technique. The atomized drop sizes, represented by either the mass median diameter (MMD) or the volume mean diameter (VMD), could be described in terms of three non-dimensional groups, namely liquid-to-air mass ratio (hi,/h*), Weber number (We), and Ohnesorge number (Z) in the simple form: MMD/D = (1 + hjL/MA)X1{x,We-"z + xqZjX2} where j = 1 and 2. The exponents and coefficients (xi, i = l-4) are determined by the best least square fit of the equation to the experimental results using the iterative generalized inverse method. Coefficients of correlation of 0.95 and 0.94 have been obtained for Newtonian and pseudoplastic liquids, respectively. The exponents of the We-'-and Z-dependences of the drop sizes fall between the values 2/3 and l/3, predicted by the acceleration and the capillary wave mechanisms. This simple equation, based on the wave mechanism, is in good agreement with the empirical models reported in the literature for airblast atomization of low-viscosity liquids. In addition, it was found that the atomized drop sizes of Newtonian liquids substantially decrease as the atomizing air pressure exceeds a threshold value. For example, at a liquid-to-air mass ratio ML/MA of 1.3, the MMD of the atomized glycerol drops decreases from 93 to 64 pm as the atomizing air pressure increases from 170 to 207 kPa. This pressure effect may be attributed to the sudden expansion at the atomizer tip where the atomizing air flows at the maximum achievable velocity (sonic velocity). No such pressure effect is seen in airblast atomization of pseudoplastic liquids which gives rise to significantly larger drops than Newtonian liquids atomized under similar conditions.