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While oxygen sensors provide the means by which changes in exhaust gas AFR (air-to-fuel ratio) are monitored and controlled in three-way catalyst systems, the chemistry of the exhaust gas in contact with this solid state electrochemical sensor can exert a substantial influence on its AFR control performance. Such interactions have been examined in a fundamental study on commercial oxygen sensors (unheated and heated), firstly using simple gas mixtures, and then simulated exhaust gas mixtures of progressively increasing complexity. The work confirms that diffusion effects at the sensor surface are centrally important in determining sensor response, but indicate that the effects of H2 (the smallest species present) do not necessarily dominate the observed behaviour. The results allow the development of a relationship that can be used to estimate the extent of the expected overall lean or rich shift for the sensor as a function of the exhaust gas composition.