Hydrogen peroxide is used in formulations to control biofouling and for disinfection purposes in technical water systems or in the medical field. In this work the efficiency of hydrogen peroxide in combination with silver- and iron ions and peracetic acid, respectively, was investigated mainly against bacteria in biofilms. Experiments were performed with pure cultures of the strain Pseudomonas aeruginosa and mixed populations of drinking water bacteria. In experiments with biofilms, grown in batch culture, it was detected that the presence of 150 mg/L hydrogen peroxide with a contact time of 24 h had neither a killing nor a detaching effect; also not with the addition of silver ions (150 µg/L). Only the addition of iron(II)ions enhanced the antimicrobial activity of hydrogen peroxide but did not influence the detachment of biofilms. The generation of hydroxyl radicals via the Fenton reaction has been proved. Biofilm bacteria always showed a higher resistance than planctonic bacteria of the batch culture. In the Annular Reactor, which was modified during this work, investigations concerning the long term treatment of biocides against biofilms as well as their resistance and regrowth were per-formed. Biofilms were grown in Annular Reactors and treated daily with the combination hydro-gen peroxide and peracetic acid over a period up to 42 days. The application of high concen-trations caused a decrease in microbial activity but no bacterial detachment. The maint enance of the dead cells on the surfaces causes a selection of single resistant organisms that can start to multiply and lead to regrowth in the test system. According to practical conditions also lower concentrations of the combination hydrogen per-oxide and peracetic acid were applied. Neither a killing nor a detaching effect was observed. It was shown that after the third biocide treatment hydrogen peroxide and peracetic acid, respec-tively, were consumed significantly; a possible explanation would be the adaptation by in-creasing the catalase activity. The direct investigation of the biofilms in the confocal laser scan-ning microscope showed a changing in the biofilm architecture caused by the biocide treat-ment. The treated biofilms seemed to be more aerated. That could have an positive effect on e.g. heat exchange systems where the tested biocides are applied. Finally the different methods for testing the efficiency of biocides were compared and evaluated concerning their relevance in practice. From the findings of this study it becomes clear that the methods for detection of the efficiency of biocides should be adapted to practical conditions. In addition to this, supplementary additional microscopic investigation can give useful on the effi-ciency of biocides against biofilms.