A novel cationic dyeable copolyester (MCDP) containing purified terephthalate acid (PIA), ethylene glycol (EG), 2-methyl-1,3-propanediol (MPD), and sodium-5-sulfo-isophthalate (SIP) was synthesized via direct esterification method. The chemical structure of modified cationic dyeable polyester (MCPD) was confirmed by FTIR and H-1-NMR. The thermal properties of MCDP and cellulose acetate butyrate (CAB) blends with different blend ratios were investigated by DSC. The results revealed that MCDP and CAB were immiscible polymer blends, and the glass transition temperature of CAB in blend fibers was higher than that of CAB in blend chips because of the strengthening hydrogen bonding. The chemical changes of MCDP and CAB in blend melt spinning were analyzed. It was found that the thermal hydrolysis reaction of ester side groups of CAB occurred in blend melt spinning, which resulted in that the acid gas was produced and the hydroxyl group content of CAB was increased. Furthermore, the moisture absorption of blend fibers was improved about three times than pure MCDP fiber even after washing 30 times. (c) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2487-2495, 2010