Using a visualization prototype built from original electrical-submersible-pump (ESP) components and with minimal geometrical modifications, a pioneer experimental procedure was developed and conducted to address the viscous effect on liquid/gas two-phase flow through these types of pumps. Based on dimensionless groups that govern centrifugal-pump single-phase performance, two-phase experiments were conducted at different shaft speeds (15, 25, and 30 Hz), with nonslip void fractions (up to 5%), and viscosity values of 46 to 161 cp, while liquid rates were kept constant at 60% of the maximum rate at the defined shaft speed. High-speed video footage was taken from the entire impeller flow channel, and stage incremental pressure was measured. The authors identified four liquid/air flow patterns inside the impeller channels: agglomerated bubbles, gas pocket, segregated gas, and intermittent gas. By comparing the images with the differential-pressure data, it was concluded that the agglomerated-bubbles pattern is responsible for the initial head degradation and that the surging event coincides with the gas-pocket structure, indicating that this is an interface-instability problem. Another conclusion made was that the increase in viscosity caused surging to occur at lower void fractions, which could be compensated for by increasing rotational speed. The significance of this work is given by the fact that several authors hae investigated centrifugal-pump performance under two-phase flow; however, previous experiments have been conducted only with water as the liquid, thus neglecting the viscous effect on the two-phase-flow mixture. In most of the petroleum industry's applications. ESPs operate with oil and natural gas. The present work begins the task of addressing this knowledge gap between scientific research and field applications.