Hydrogen peroxide (H₂O₂) belongs to the reactive oxygen species (ROS). In low concentrations (nM to low µM), it displays intra- and extracellular signaling functions, mainly by oxidizing target molecules, like proteins and lipids. Likewise, yet in much higher concentrations (high µM to mM), H₂O₂ is employed by specialized cells to kill pathogens, like bacteria. Cellular production of H₂O₂ is paralleled by effective H₂O₂ degradation due to a multitude of enzymatic systems. Thus, balanced intracellular [H₂O₂] levels are obtained, guaranteeing physiological functioning of the cell. Using a scanning electrochemical microscope (SECM) and platinum ultramicroelec­trodes, basal and stimulated H₂O₂ production and concomitant degradation from single primary human monocytes were measured in real-time and with high temporal resolution (1 Hz). Electro­chemically produced signals can be quantified for each measuring point, and thus enable insight into the dynamics of [H₂O₂] changes over time and under different patho-/physiological conditions.