This paper presents experimental data and a preliminary analysis on the operation of centrifugal pumps with viscous liquids. Two centrifugal pumps, a conventional radial (Ns = 1000) and a semi-axial electrical submersible pump - ESP (Ns= 3850) were instrumented and tested with water – 1 cP, and clear glycerin. The glycerin viscosity was varied from 67 cP to 1020 cP, through changes in temperature, encompassing the viscosity range of light to heavy oils. The main purpose of these tests, besides measuring the influence of viscosity upon the pump overall performance, was to supply detailed information on the energy transfer processes taking place in the pump internal components. To accomplish with that the pressure distribution along the flow path, through impellers and diffusers, from the pump inlet eye to the discharge section, was measured. Thus, besides measuring the flow rate, the total pressure difference, the speed, the power and the mean operation temperature, for a working liquid with various viscosities in a full range of operational conditions, the data showed the pressure evolution inside the pump. Later analyses revealed that there was a strong relationship between the flow hydrodynamics in successive pump devices in terms of head gain and viscous dissipation. This relationship was set by the pump operational operational conditions, flow rate and speed, and the fluid viscosity. In other words, how a diffuser performed depended on Reynolds number and the upstream flow through the impeller. Thus, any generalizing model dealing with the influence of fluid viscosity on the pump performance should account for these phenomena. For this reason, it is highly improbable that "black-box" approaches that neither consider the flow hydrodynamics of the successive connecting devices nor assess the influence of the Reynolds number on them could give proper viscosity correction factors for different pumps.