The mixing process in a Lightnin Series 45 static mixer, 40 mm in diameter, has been investigated using computational fluid dynamics and the volume of fluid (VOF) method, a method developed for immiscible fluids but here used for miscible. The mixing process was investigated for two liquids that had viscosities ranging from 0.003 to 51.2 Pa s, and the volumetric flow rate proportion between the liquids was varied between 1/1 and 4/1. The flow rate was 0.1 m/s, and two Reynolds numbers, I and 70, were investigated. The mixer performance was evaluated using the rate of striation thinning, and it was found that the greater the difference in viscosity the worse the mixer performance. This effect is due to the difference in elongation rate between the liquids, which exists until equilibrium in shear stress has been reached at the interface between the liquids. For higher Re numbers close to a point when secondary motions start to have significance for the rate of striation thinning, a lower viscosity of the added liquid results in an increase in mixing performance. It was further found that the VOF method can be used to model the mixing of dissimilar liquids in static mixers, but since the thickness of the striations decreases rapidly with the number of mixer elements, the VOF method is most suitable when investigating mixing processes over a small number of mixer elements.