The sections in this article are Introduction Theoretical Foundations of Gas Potentiometry Physico‐Chemical Measuring Principle Solid Electrolytes Resumé GOP Applications in Research and Industry Materials, Design, and Systems Sensor Materials GOP Designs Electrical Metrology Response Time Analysis and Characterization of Gaseous and Liquid Fuel Combustion In Situ Measurement On‐Line Measurement (Off‐Flame Gas Potentiometric Measurement) Analysis and Characterization of Solid Fuel Conversion Gas Potentiometric Measurements in Combustion and Gasification Chambers Burn‐Out Characteristics of Solid Fuels, Biofuels, and Waste Materials under Combustion and Gasification Conditions Gas Potentiometric Combustion and Gasification Analysis: Burn‐Out Characteristics and Fuel‐Specific Makrokinetic Parameters Modeling to Determine Fuel‐Specific Makrokinetic Parameters Resumé Applications with Potential for Development The Performance of a Velocity‐Oxygen Probe Measurement of Fluid Dynamics in a Fluidized‐Bed Reactor Feed Control in Solid Fuel Gasification O 2 Concentration Distribution in a Fluidized‐Bed Membrane Reactor Outlook Conclusions