Objectives: To determine the disinfecting effect of ozonized oxygen (120 seconds from the HealOzone generator, KaVo) on Enterococcus faecalis, representing bacteria that are difficult to eliminate in the root canals of human teeth, and to compare it with the conventional irrigants: sterile physiologic sodium chloride solution (negative control group), 3% hydrogen peroxide solution, 0.2% chlorhexidine solution, 1.5% sodium hypochlorite solution, and 3% sodium hypochlorite solution (positive control group). Method and Materials: The roots (n = 10 in each group) were sterilized, contaminated with the test microorganisms in a quantitative preparation, rinsed with the test solutions, and dried. The residual concentration of E faecalis was determined through another irrigation stage with the sodium chloride solution. Results: The positive control group showed a significantly lower concentration of microorganisms than all the other groups, whereas the negative control group showed a significantly higher concentration compared to the other groups. The test groups showed low concentrations. Conclusion: Ozonized oxygen appears to be suitable for disinfecting root canal systems in cases where sodium hypochlorite is not indicated. Keywords: chlorhexidine, ozone, root canal cleaning, sodium hypochlorite