Microorganisms are omnipresent, some of them are advantageous, others are pathogenic germs. Pathogenic microorganisms in food are of cause unwanted, because they can make people fall ill or at least turn the foodstuff unappetising by changing colour, appearance or smell. Hence fresh food either has to be consumed shortly or it has to be preserved. Usually food is preserved by heating treatment, this technique kills microorganisms reliable, but simultaneously, there is a loss of vitamins in the foodstuff. Also often a change in colour and the formation of roasting flavours happens. Therefore product gentle preservation techniques are looked for. In the presentation results of the treatment of microorganisms with supercritical carbon dioxide at high pressure are shown. Carbon dioxide is compressed and is admixed at pressures of up to 150 MPa and at a temperature of 40°C to the liquid to be sterilized. After that, the mixture is expanded rapidly to ambient pressure. Subsequently the residual number of surviving microorganisms is analysed by plate count. Experiments were carried out with Escherichia coli, Lactobacillus acidophilus, Saccharomyces cerevisiae and also with activated sludge from a sewage plant. At least 6 orders of magnitude of the different species of microorganisms were inactivated in this single process step. Beyond the results, a new hypothesis on the process of inactivation is suggested.