This paper describes in a very practicable way the operating conditions of the iodide-iodate reaction system used to quantify micromixing phenomena in chemical reactors and mixers. Several operating procedures are given for batch and continuous mixers. Recommendations to adapt these procedures to any specific mixers and complete analysis procedures are also provided. Finally, a simple micromixing model is used to calculate micromixing time t(m). It is checked that t(m) is non-dependent on the reactant concentrations at constant hydrodynamic conditions. In a second following paper (Part II), the reactions kinetics, essential for a knowledge of micromixing time t(m), are presented.