In the description of surface-catalysed reaction two different concepts are mainly discussed: the Eley-Riedel mechanism and the Langmuir-Hinshelwood mechanism. Experience has shown that the latter is the most important one. According to this concept, the educt species can only react with one another if both types of molecules are simultaneously adsorbed on the catalyst surface. In order to describe this adsorption state, frequently, the adsorption model according to Langmuir is used. This concept assumes, in particular, that the coadsorbed molecules have nearly the same molecular size. In practical problems, this assumption only seldom holds. In order to investigate the influence of the size of the adsorbed molecules on the reaction rate of a given surface-catalysed reaction, an adsorption concept, recently proposed as the so-called CS-model, can be taken. Using a simple example, the results obtained with these different concepts are discussed in detail.