In recent years, there has been an increased use of polymers to modify asphalt binders, mainly to decrease pavement rutting but also to improve binder failure strain in direct tension. With binder oxidation, however, failure strain improvements (relative to the unmodified binder) disappear, and two mechanisms are suspected: polymer degradation and base binder embrittlement. The extent to which each of these mechanisms is responsible for loss of modified-binder performance is important and bears on maintenance treatments and recycling methodologies. Changes in styrene-butadiene-styrene (SBS) polymer-modified binder properties due to oxidation were analyzed by means of dynamic shear rheometry, ductility, force ductility, and gel permeation chromatograph. Previous literature reports using size exclusion chromatography showed that degradation of the molecular weight profile of SBS accompanied the loss of ductility in polymer-modified asphalt (PMA). Yet base binder embrittlement also occurred, as evidenced by ductility and force ductility. Testing aged PMA binders at higher temperatures to soften the base binder restored the polymer modulus to the force ductility measurements, as did blending with a softer deasphalted oil. The conclusion from these measurements is that the primary cause of PMA degradation with aging is base binder embrittlement rather than polymer degradation.