This paper presents an experimental study about the performance of a centrifugal pump, focusing on the effects of the fluid viscosity and the intake free-gas fraction. The first part evaluates a single-phase performance map proposed by Solano (2009). By use of dimensionless numbers, this map enables us to determine the pump performance when handling viscous fluids as a function of rotational speed at operational points other than the best efficiency point. Additionally, while the performance map of Solano (2009) is built solely on experimental data, this article expands his work by use of polynomial correlations that are based on basic phenomena that enable the prediction of the pump performance under a wider range of conditions. It was observed that these polynomial correlations are able to represent--with good agreement--the head, the brake horsepower, and the efficiency of a specific pump. Most of the work on multiphase flow through electrical submersible pumps has been performed with water as the liquid,. Unfortunately, this does not correspond to actual petroleum liquids, which many times present viscosities a hundred times greater than that of water. In view of this, the second part of the present study focuses on the pump performance when handling a highly viscous fluid under two-phase flow. An extension of the application of the dimensionless numbers is proposed to fit two-phase-flow data for the same pump.