Continuous surface treatments of PET and PTT fabrics were carried out to modify their dyeability to four cationic dyes using an electrodeless UV bulb for different periods of time. ESCA analysis of the irradiated fabrics showed higher O/C ratio than that of the untreated fabrics indicating photooxidation of the outer surface layer. PET fabric was photo-oxidized more readily than PTT, while color change was more pronounced in the PTT fabric compared with PET fabric as shown by larger yellowness increase and color difference values. Four cationic dyes, including two hemicyanine dyes and two triphenylmethane dyes, were used to assess the dyeability to the modified polyester fabrics. The irradiated fabrics became more dyeable to cationic dyes with increasing treatment time due to the electrostatic or acid/base interactions. While increase in cationic dyeability was more noticeable in the treated PTT fabric, overall dyeability to cationic dyes was higher in the case of the PET fabrics as expected. Among the dyes used, the triphenylmethane dyes showed higher dyeability to the cationic dyeable polyester fabrics compared with the hemicyanines. Surprisingly laundering and rub colorfastness of the polyester fabrics dyed with cationic dyes were good even though the treatment may only change the surface layer of the fabrics.