The factors affecting the speed of response of solid electrolyte gas sensors are considered. The uptake or release of sample material (gas) by such cells is the main reason for sluggishness in response and for variation with time in the emf developed. Special attention is given to the effect on the emf caused by the presence of a second phase with electrical properties different from those of the main material. For a model cell, consisting of a solid electrolyte containing an intergrain phase with mixed conductor properties, the diffusion equation is solved numerically. The ambipolar diffusion coefficient is used in the calculations. The numerical parameters required for the calculation of concentration profiles and emf as a function of time are those of a typical calcia‐stabilized zirconia. The results have a more general validity.