Three kinds of modified poly(ethylene terephthalate) copolyesters were synthesized, using sodium-5-sulfo-bis-(hydroxyethyl)-isophthalate as the third monomer, 1,3-propanediol (PDO), 2-methyl-1,3-propanediol (MPD), and 2,2-dimethyl-1,3-propanediol (neopentyl glycol or NPG) as the fourth monomer, respectively. The copolyester fibers were also prepared by melt spinning and drawing processes. The effect of PDO, MPD, and NPG on the synthesis and spinning process was investigated, and the structures and properties of both copolyesters and the produced fibers were characterized. The results exhibited that the structural difference of PDO, MPD, and NPG played an important role in the synthesis and spinning process, and significantly affected the structures and properties of both copolyesters and the produced fibers, which thereby resulted in the difference in terms of dyeability improvement of copolyester fibers. The dyeing at boiling temperature under normal pressure experiments of copolyester fibers in both disperse dyebath and cationic dyebath revealed that incorporation of the fourth monomer could improve the dyeability of copolyester fiber, and copolyester fiber containing MPD unit had better dyeability due to a looser, more accessible structure when compared with the fiber containing PDO or NPG unit. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013