The tensile properties of polyether-based polyurethane (PU) filaments decrease with increasing chlorine concentrations as well as with treatment times. Fourier transform infrared (FTIR) results show the formation of quinoid, ate, and aldehyde groups in the chlorine-treated PU, and increased hydrogen bonding between the C-O-C in the soft segment and the N-H in the hard segments. A breakdown mechanism involving chain cleavages along the ether linkages in the soft segments as well as at the urethane linkages of the hard-soft segment interfaces is proposed. Chlorine-treated PU showed increased solubility in tetrahydrofuran (THF). The molecular weight data of the THF-soluble portion of treated PU also support the proposed locations of chain scissions. The increased soft segment T-g and T-m with increasing chlorine concentrations are results of increased phase-mixing and hydrogen bonding.