An indoor pollution of polychlorinated biphenyls (PCB) of about 4000 ng/m3 could be reduced by an average amount of 68% by temporary sealing of the primary source in combination with intensive cleaning. The costs were around DM 100,000.- for the school building containing 12 classrooms. Due to this procedure a continued usage of the building is possible until a concept of decontamination is available. In another building the PCB indoor pollution was reduced by 73% in a representative trial restoration, whereas the stripping of the paint with a chemical caustic substance induced a significant reduction of the PCB concentration. The usage of special air conditioners--called "Luftwäscher"--in the classrooms did not reduce the PCB concentration. The realisation of disturbing factors such as the fluctuation of the temperature is of great importance for judging the efficiency of decontamination procedures. The possible misinterpretation regarding the results and the data of decontamination procedures can be excluded by parallel measurements in control rooms. Costs induced by these additional analyses will be reduced by the early realization of ineffective steps.