Experimentally synthesized poly(1,4-butylene terephthalate-co-tetramethylene oxalate) (PBT-PTMO) monofilaments were evaluated for hydrolytic stability in salt water (SW) and distilled water (DW) at temperature below and above glass transition temperature (T g ), along with commercially available poly(hexamethylene adipamide) (NY), poly(ethylene terephthalate) (PET), and polypropylene (PP) monofilaments. There was no decrease in mechanical properties in case of NY, PET, and PP in either DW or SW below their T g . The breaking strength, ultimate elongation, and thermal shrinkage of the PBT-PTMO, however, decreased as the ageing time increased. Total strength loss occurred after approximately 300 days at 25ยฐC in either DW and SW. This can be attributed to the chain scission that occurs in the PBT-PTMO copolymer chain. The poor hydrolytic stability of the PBT-PTMO may be attributed to the higher moisture regain. The salinity of water did not have a significant effect on the breaking strength loss of the materials. The mode of hydrolytic degradation of aged PBT-PTMO polymer was confirmed by the increasing generation of the acid carbonyl and hydroxyl groups with concomitant increasing consumption of ester groups, regardless of ageing conditions. Above T g , the hydrolytic rate constant (kะ H , day ฯช1 ) of the PBT-PTMO, estimated by the rate of formation of acid carbonyl groups, is greater at a higher ageing temperature.