This paper presents a new method of controlling the air-to-fuel ratio (A/F) in gas engines with three-way catalysts. In the method under discussion, the center of the λ-window is detected directly through the dynamic response of an O2 sensor installed at the rear of the catalysts. As the result, the A/F remains stable at the center of the λ-window for long periods of time, even in cases where static response characteristics of the O2 sensor change due to the deterioration of the O2 sensor, or in instances where the λ-window shifts or decreases due to the aging of the catalysts.The responses of the O2 sensor outputs have delay times in both the step changes of lean-to-rich and rich-to-lean A/F. Varying delay times were observed, and it was determined that the differences depended on the mean value of the A/F of the step change. Moreover, the delay times equalized when the mean A/F was at the center of the λ-window. These phenomena are considered to be caused by O2 storage, CO adsorption, HC retention, and the water shift reaction of the catalysts. When the A/F is modulated sinusoidally, these phenomena appear as symmetrical response waveforms in the O2 sensor outputs. The waveforms are asymmetrical when the mean A/F is not centered in the λ-window. Therefore, the A/F can be controlled at the center of the λ-window by feedback operation that keeps the response waveforms symmetrical.Experiments with new control method demonstrated that less than 80, 150, and 150 ppm emissions of NOx, CO, and CH4, respectively, were achieved even for older catalysts. This method is easily applied to conventional gas engine controllers.