In a small water treatment system, electrochemically produced mixed oxidants by electrolysis of brine have shown promise as an alternative disinfectant to free available chlorine (FAC). However, the efficacy and mechanisms of electrochemically mixed oxidants as a disinfectant for microorganisms such as E. coli and Bacillus subtilis spores have not been reported widely, and those that have been reported are controversial in some aspects, particularly in comparison with FAC. Hence, this study was performed to evaluate the efficacy of electrochemically mixed oxidant solution as a disinfectant in comparison with that of FAC solution on the basis of an equal weight of the total oxidants per unit volume. In addition, the chemical characteristics of the electrochemically mixed oxidants was examined by evaluating the disinfection efficiency of the mechanically mixed oxidants, which were made by adding small amounts of subsidiary oxidants such as O₃, ClO₂ and H₂O₂ to FAC. The major conclusions are as follows: firstly, electrochemically mixed oxidants are approximately 20-50% more efficient in inactivating E. coli and Bacillus subtilis spores at certain pH conditions (pH 8.2) than FAC. These observations can be explained by the characteristics of the oxidants, such as HOCl, O₃, ClO₂, H₂O₂, which would be generated by the cathode reduction of oxygen and the anode oxidation of chloride ion in the electrolytic cell of brine. However, the exact composition of the liquors produced from the electrochemical processes is not well understood because of the lack of selective analytical methods for detecting the specific oxidants. Secondly, the enhanced disinfection efficacy of mechanically mixed oxidants was observed in comparison with FAC as they were made by adding a small amount of O₃ or ClO₂ to FAC. These improved results were explained by the effect of chlorine dioxide and chlorite which can possibly be generated as a result of reaction with an excess of chlorine and a small amount of ozone generated in electrolytic cell. Includes 30 references, tables, figures.