A series of anionic copolyester/dodecylbenzenesulfonate (DBS) blend films were prepared by a melt-compounding method and then annealed under a range of conditions. To eliminate possible experimental errors in measuring the alkaline reduction rate of the samples, the crystallinity should be uniform. We used Fourier transform infrared (FT-IR) spectroscopy to measure and control the relative crystallinity of the annealed samples. The relationship between the crystallinity and reduction rate, as well as the annealing conditions of the annealed blend samples, was derived. FT-IR analysis showed that the crystallization rate of copolyester/DBS blend films is lower than that of pure copolyester. However, the alkaline weight reduction rate of the blend films, which have similar crystallinities, increases dramatically with increasing DBS content, even at a low DBS content of 1 to 3 wt%, although the effect of the DBS becomes negligible above 5 wt%. However, unlike the result for annealed films, the alkaline weight reduction rate for amorphous films exhibited a linear relationship between the alkaline reduction rate and the DBS content for all of the examined DBS content, demonstrating that the effects of the DBS on the alkaline weight reduction of the copolysester could be affected by processing conditions such as post-drawing and annealing.