Cationic dyeable polyester and poly(butylene terephthalate) fabrics were treated with ozone gas under atmospheric pressure and at a pressure of 0.1 MPa. The effect of the ozone-gas treatment was investigated on the basis of electron spectroscopy for chemical analysis, Fourier transform infrared spectrum, X-ray diffraction, moisture regain, water absorption, and dyeing properties. The ozone-gas treatment caused an increase in the oxygen content of each fiber. From the wave separation of C-1s, the oxygen was incorporated in the form of -CO- and -COO- on the fiber surface, and also, despite an increase in the water absorption, the fiber density as an index of crystallinity increased a little with the treatment. As the result, we concluded that the treatment caused a change not only on the fiber surface but also in the internal structure of both fibers. Then, it seemed that the treatment brought about an effect on the dyeing properties. The apparent dyeing rate with the cationic dye increased remarkably with the ozone-gas treatment, whereas that with the disperse dye increased slightly. Therefore, it was clear that the ozone-gas treatment caused a change not only in the fine structure but also in the dyeing behavior. These phenomena were the same as those of poly(ethylene terephthalate) fibers. (c) 2007 Wiley Periodicals, Inc.