Zhang (2006) utilized computational fluid dynamics (CFD) to examine the validity of erosion models that have been implemented into CFD codes to predict solid-particle erosion in air and water for inconel 625. This work is an extension of Zhang’s work and is presented as a step toward obtaining a better understanding of the effects of fluid viscosity and sand-particle size on measured and calculated erosion ratios, where erosion ratio is defined as the ratio of mass loss of material to mass of solid particles. The erosion ratios of aluminum 6061-T6 were measured for direct impingement conditions of a submerged jet. Fluid viscosities of 1, 10, 25, and 50 cP and sand-particle sizes of 20, 150, and 300 μm were tested. The average fluid speed of the jet was maintained at 10 m/s. Erosion data show that erosion ratios for the 20- and 150-μm particles are reduced as the viscosity is increased, whereas, surprisingly, the erosion ratios for the 300-μm particles do not seem to change much for the higher viscosities. For all viscosities considered, larger particles produced higher erosion ratios, for the same mass of sand, than smaller particles. Concurrently, an erosion equation has been generated based on erosion testing of the same material in air. The new erosion model has been compared to available models and has been implemented into a commercially available CFD code to predict erosion ratios for a variety of flow conditions, flow geometries, and particle sizes. Because particle speed and impact angle greatly influence erosion ratios of the material, calculated particle speeds were compared with measurements. Comparisons reveal that, as the particles penetrate the near wall shear layer, particles in the higher viscosity liquids tend to slow down more rapidly than particles in the lower viscosity liquids. In addition, CFD predictions and particle-speed measurements are used to explain why the erosion data for larger particles is less sensitive to the increased viscosities.