The application of alkaline hydrolysis to study the change in the fine structure of bicomponent polyester fibers as their surface is removed progressively was explored. The samples were prepared with a poly(butylene terephthalate) (PBT) sheath and a poly(ethylene terephthalate) (PET) core. The reagent used to hydrolyze the PET was 1M NaOH in 75/25 methanol to water since it appeared to react topochemically with the fiber. The solution reacted more rapidly with PET than with PET. Thus, when necessary to retard the weight loss of the bicomponent fibers, after a 2-h hydrolysis with this reagent to remove PET, it was replaced with aqueous 1M NaOH solution containing 0.1% cetrimmonium bromide. Unlike homofil PET or PET fibers, where alkaline attack appeared to be confined to the surface and left the residue relatively smooth, the bicomponent fiber was attacked unevenly, and penetration to the PET core occurred before all the PET at the surface was removed. Nevertheless, most of the reaction was confined initially to the PET sheath. The tenacity and extension at break of the PET-PET fiber passed through a maximum as hydrolysis progressed. The fall in tenacity at high weight losses is ascribed to increasing surface defects in the fiber surface. After removal of the PET by the hydrolysis, the birefringence of the residue became progressively higher. The synergistic effect of the PET sheath on the properties of the PET core and the possible causes of the nonuniform hydrolysis at the PET surface are discussed. An equation is proposed that includes an interaction parameter, which can be utilized to determine which property is affected most by the hydrolysis of a bicomponent fiber. In this instance, it appears from the parameters that the order is strength > extension at break approximate to birefringence.