A study on the influence of the gas testing systems in the behavior of solid-state electrochemical exhaust gas sensors is presented. Several YSZ-based exhaust gas sensors have been tested in different versions of gas testing chambers, with different architectures, volumes, exhaust gas flow dynamics, temperature regulation, etc. Sensors have been tested in synthetic gas testing chambers and in real combustion systems. Results show that parameters such as the response time or even the equilibrium open circuit voltage may be strongly dependent on the characteristics of the testing system. Moreover, a lack of gas flow homogeneity or a wrong temperature distribution in the testing chamber may dramatically influence the behavior of exhaust gas sensors. Optimum designs of experimental systems are presented in terms of the sensing characteristics to be tested.