The preparation of cavities with oscillating instruments facilitates optimal formation of an exact bevel for composite fillings especially in smaller defects. The aim of the examination was to determine whether side effects of the sonoabrasive SonicSys®-System causes zones of debris or cracks in enamel and dentin. As standard of comparison conventional rotating instruments should serve. 11 freshly extracted, caries-free teeth were fixed in a plaster-block between 2 phantom-teeth. Mesial and distal small cavities were prepared with a diamond bur (ISO 830 010). The margin of one cavity was finished conventionally with a flame shaped bur, (ISO 8830 009) in an angle of 45°; the other cavity was smoothed with the SonicSys®-approx-instrument. The 22 cavities were filled accordingly to the manufacturers advices with the composite system Syntac®/Tetric®. Afterwards the teeth were fixed in formalin and embedded in a light-curing plastic medium. For the histological examination undecalified cuts of 10 µm thickness were prepared from the se samples. The comparative light-microscopic evaluation of the samples showed no differences in dependence on the chosen finishing instruments in reference to the margin quality and the adjoining tissue-layers. Zones of debris and cracks were not verifiable in dentin and enamel. In reference on the sealing quality no differences were to be proven between the two examined systems. In this study oscillating vibrations of the SonicSys®-Systems did not cause verifiable side effects in the area of the cavity walls. We are not expecting an impairment of the quality of the restoration.