Despite the common presence of water in oil production, just recently, the scientific community has devoted efforts to studying the influence of emulsion phenomena effects related to oil production using pumps. In the context of this study of phase inversion phenomena, the influence of viscosities and rotational speeds in electrical submersible pumps (ESPs) are evaluated as part of this effort. This study is aimed at investigating the influence of viscosity in phase inversion phenomena. An eight-stage ESP was tested with three different rotational speeds and two different oil viscosities for the best efficiency point (BEP) flow rates. Initially, the total flow rate was obtained in relation to BEP using ESP performance curves for pure oil at 52 cP and 298 cP and rotational speeds of 800 rpm, 1200 rpm, and 2400 rpm. The total flow rate was kept constant and the water cut was increased from 0 to 100%. The inversion phase phenomenon was detected by performance improvement when the water cut increased. The factors analyzed were the head and efficiency of the ESP as a function of the water cut. The phase inversion experimental data obtained in this study were compared with literature models for horizontal pipes. The results of this comparison presented satisfactory agreement. The phase inversion phenomena occur in all eight stages at the same time. Hysteresis was observed in ESPs for oil viscosity of 52 cP and rotating speed of 800 rpm and 1200 rpm.