The kinetics of inactivation by potassium ferrate were studied using a bacteriophage, F-specific RNA-coliphage Qβ as a viral model. The inactivation appeared to be expressed by Hom's model in phosphate buffer at pH 6, 7, and 8. The rate of inactivation depended on pH; the lower pH, the faster inactivation observed. To consider the mechanism by which ferrate caused inactivation, the efficiency of inactivation was checked after ferrate decomposition in buffer. Effective inactivation following Hom's model was also observed after the complete decomposition of ferrate ion; however, the efficiency of that inactivation disappeared by the addition of sodium thiosulphate, suggesting that rather long-lived oxidative intermediate was generated by the decomposition of ferrate ion. The intermediate might take part in the inactivation.