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One of the main problems in the clinical use of silicone oil in vitreoretinal surgery is the instability of the material in terms of emulsification. Fine silicone oil droplets can cause a secondary glaucoma by blocking aqueous outflow. Intravitreally applicable silicone oils comprise an inhomogeneous group of materials, and the rate of emulsification depends on the physicochemical properties of the silicone oils. Clinically most frequently used silicone oils are highly purified polydimethylsiloxanes with a viscosity of 1000 centistokes (cs) to 5000 cs. Low-viscosity silicone oils are preferred by some surgeons because of easier surgical handling and easier removal out of the vitreous. The authors investigated highly purified polydimethylsiloxanes in viscosity ranges of 1000 cs, 2000 cs, 3000 cs, 4000 cs, and 5000 cs with regard to their in vitro stability to evaluate the optimal range of viscosity with acceptable material stability. The comparative stability tests were performed with 0.1% salt solutions of albumin, acidic alpha-1-glycoprotein, fibrin, fibrinogen, gamma globulins, and very-low-density lipoprotein as emulsifiers. Silicone oil at 5000 cs was in all cases distinctly more stable than the silicone oils with a viscosity up to 4000 cs. A positive correlation between the degree of viscosity and an increase of material stability was not found with all emulsifiers. When very-low-density lipoprotein, albumin, and fibrin served as detergents, a remarkable stability gain was achieved only at 5000 cs. With fibrinogen, acidic alpha-1-glycoprotein, and gamma globulins, a positive correlation between the elevation of viscosity and increased stability was observed in the intermediary viscosity ranges from 2000 to 4000 cs, with an additional gain at 5000 cs. According to these results, the material stability of purified polydimethylsiloxanes increases remarkably at 5000 cs compared to silicone oils of lower viscosity. This behavior might be explained by a desultory increase of stabilizing intermolecular forces in the viscosity range of 5000 cs. Hence, purified 5000-cs polydimethylsiloxane is recommended for intravitreal application to keep silicone oil emulsification side effects as low as possible.