A correlation between the yield behavior of copolymers based on poly(ethylene terephthalate) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) and their secondary relaxation motions is established. The yield stress decreases as the cyclohexylene content increases for different temperatures and strain rates. The activation volume based on Eyring＇s model of yielding increases as the copolymers become more PCT-like, while the activation energy does not exhibit any significant change. We speculate that the conformational changes of the cyclohexylene rings reduce the barriers between chain segments sufficiently to facilitate chain slippage. This is supported by the increased dynamic fluctuation found by positron annihilation lifetime spectroscopy. The crazing stress of the copolyesters increases with increasing cyclohexylene content. We propose that the formation of stable microvoids is impeded by local stress relaxation; hence craze formation is retarded. The ductile/brittle transition is viewed as a competition between yielding and crazing with changes in the transition temperature dependent on activation of molecular motions of the cyclohexylene groups.