Post-extrusion solid-state polymerization (SSP) of a commercial fully drawn filament yam (FDY) of poly(ethylene terephthalate) was carried out at 220 degrees C, 230 degrees C, and 240 degrees C for a duration of 30 min to 2 h under inert atmosphere. Molecular weight of the solid-state olymerizecl polyester filaments was increased from 1.67 x 10(4) gm/mol to a maximum of 2.61 X 10(4) gm/mole for the sample subjected to 240 degrees C for 2 h. The kinetics of the SSP in the highly oriented crystalline FDY polyester filaments was investigated using an empirical relation between initial molecular weight and time of SSP and was found to be greatly enhanced, compared to amorphous unoriented polyester chips. Though the free annealing (i.e., under no tension) of samples at high temperature during solid-state polymerization had a detrimental effect on the orientation of the FDY yam, the simultaneous increase in the molecular weight compensated the loss in mechanical properties to a great extent. Application of tension during SSP was found to improve the mechanical properties of the SSP yam by a small value. (c) 2006 Wiley Periodicals, Inc.