Electric submersible pumps (ESPs) are currently being tested in the service company's newly built viscous lab testing loop; whereby, the viscous performance of each pump stage type is analyzed at multiple speeds and viscosities. This viscous lab testing loop is capable of speeds up to 70 Hz, 30,000 B/D of fluid, with a maximum of 500 HP, and can manage fluid viscosities up to 2,500 cP. Four pumps were tested at speeds ranging from 40 to 70 Hz, in 10 Hz intervals, and up to 2,500 cP fluid viscosity. The viscosity correction factors, for each operating speed and viscosity, were developed for each pump stage and compared with the stage-specific speed. The effects of viscosity and operating speed on flow, head, and horsepower were observed. An increase in viscosity deteriorates pump performance, while an increase in rotational speed reduces the rate of deterioration; therefore, increased operating speeds improve the performance of an ESP pump in viscous applications. Additionally, diameter and specific speed effects (a "dimensionless[DL1] " number for pump design criteria) on viscosity are being investigated as a design criterion. A correlation is being developed for the effects of the specific speed on the viscosity correction factors. The new correlation will help obtain viscosity correction factors for other pumps with different diameters, operating at different speeds and viscosities, based on the specific speed relationship. The service company's Research and Development department's viscosity testing program, its results, analysis, and new findings are discussed in this paper.