For the control of three-way catalytic converters (TWCs), simplified models of not only the TWC but also the lambda-sensors upstream and downstream of the TWC are required. The accurate interpretation of the sensor signals makes the correction or online identification of model parameters possible. Thus, a precise estimate of the species that mainly drive the sensor's output is indispensable. To gain insight into the main reactions driving the TWCs' behavior, a model derived from the literature has been applied and adjusted to measured exhaust gas transients. These were obtained from fast gas measurement devices, where all components have been measured with T-90 < 30 ms. Apart from the components that are under the regime of legislation, such as NO, CO, and HC, also species that significantly influence the lambda-sensor signal, such as H-2 and O-2, have been measured. The model agrees well with the measurements and can thus be used as a starting point toward control-oriented (simplified) modeling of TWCs.