The features of Cs enhanced oxidation of silicon is studied with the quases-Tougaard method. It is found that Cs oxide forms islands on the Si(111) 7×7 surface for exposure up to eight cycles (1 cycle=1 ML Cs+100 L O2). These islands in turn form islands of SiO2 on the surface after annealing. The surface coverage of Cs-oxide islands and silicon-oxide islands is found to be the same. At higher (≥10 cycles) exposures, the Cs oxide as well as the SiO22 layer covers the surface completely. It is also shown, that the catalytic effect of Cs is local and that the amount of produced SiO22 is directly proportional to the amount of Cs.