In this paper, a rotating rod electrode disk is introduced as a hydrodynamic electrode and both the electrolyte flow hydrodynamics as well as mass transport towards the disk were investigated and compared with the rotating disk electrode. The main difference to the rotating disk is the absence of disk insulation leading to slightly enhanced mass transport towards the disk surface. Based on a combination of experiments and mathematical modelling for a broad range of kinematic viscosity values of the electrolyte solution, a correction of the Levich equation was introduced. This enables recalculation of the data determined using the rotating rod disk electrode so that they are equivalent to those of the rotating disk electrode. Although glassy carbon was used as the material of the rotating rod electrode in this work, in principle any other suitable material can be used. Thus, it can be concluded that the rotating rod electrode disk represents a cheap and effective tool easily applicable for measurements with controlled electrolyte convection in systems that are hardly manageable using a conventionally constructed rotating disk electrode. Good examples are melts or electrolyte solutions at temperatures above 80 degrees C. The applicability of the rotating rod electrode disk was successfully tested in the continuation (Part II) of this study. There, the thin-film modified (activated) rotating rod electrode disk was used for determining the mass transport properties of the oxygen and kinetic constants of the oxygen reduction reaction in concentrated phosphoric acid at 160 degrees C. (C) 2017 Elsevier Ltd. All rights reserved.