Elastic filaments have been produced by melt spinning poly(tetramethylene terephthalate)/poly(tetramethylene ether glycol)-terephthalate (PTMT/PTMEG-T) copolymers of various hard segment contents (HSC). Some of these copolymers are difficult to spin because of their elastic nature and sluggish crystallization kinetics. Differential scanning calorimetry studies show that both the crystallization rate and temperature decrease as HSC decreases. When spinning into air, the filaments low in HSC do not crystallize on-line and are too tacky and soft to be taken up with a conventional constant tension winder. The elastic nature of these filaments prevents them from being taken up at speeds high enough for “stress-induced crystallization” to cause appreciable crystallization on-line. Spinning into a water bath allows the filaments to be taken up without a “sticking” problem. The microstructure and mechanical properties of the water-quenched filaments were found to be strongly dependent on composition and processing conditions. Small-angle and wide-angle x-ray techniques (SAXS and WAXD) indicate that the hard segment domains are lamellar-like crystallites which become preferentially oriented perpendicular to the fiber axis at high spin-draw ratios. As HSC decreases, the lamellae become more widely spaced. High levels of crystalline orientation can be produced using the water quench technique without creating appreciable levels of amorphous orientation. These are desirable features for obtaining high elastic recovery. Birefringence results indicate that these filaments continue to crystallize for up to 10 minutes after spinning. The relative degree of phase separation (DPS) decreases as HSC increases, but the actual level of crystallinity exhibits a maximum with respect to composition. Not coincidentally, the filaments that reach the highest crystallinity are the easiest to spin. The modulus of these filaments depends primarily on HSC and DPS, while the tenacity and ultimate elongation depend more on the degree of orientation developed during spinning. Elastic recovery increases as HSC decreases and as crystalline orientation increases.