An experimental study of the influence of atomizer dimensions and injection conditions on discharge coefficient and spray angle is presented. Simplex pressure-swirl nozzles atomizing heavy oil have been examined. Twenty nozzles of different geometries have been investigated, including orifice diameters down to 0.45 mm. Inviscid theory results or published correlations valid for larger atomizers do not seem to apply for those of smaller dimensions, for which viscous effects are thought to play a major role. This role is analyzed, and new correlations are proposed for discharge coefficient and spray cone angle.