Kinetics of thermal degradation occurring on polyester containing cationic dyeable comonomer units viz. 5-sulphoisophthalate moieties are studied by measurement of changes in intrinsic viscosity and carboxyl values on the polymer after subjecting the polymer chips to temperatures in the range 275-285 degrees C for different residence times ranging from 5 to 60 min and comparing with the homopolymer. The activation energy values for degradation are estimated from the kinetic data. Mechanical properties of the textured yarns produced from the partially oriented yarns (POY) spun under different residence times are measured. Yarn produced with higher residence time has poor mechanical properties. The SEM images of the POY show presence of particles at the surface of the yarn due to polymer degradation. The size of the particles as seen on the surface increase with increase of retention time. Addition of thermal stabilizer helps in controlling the thermal degradation. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2791-2800, 2010