This work concerns the determination of distillation boundaries. This is done comprehensively, using a bifurcation study for a single feed column. The approach used is to track all of the pinch points of rectifying and stripping profiles, mapping the branches of solutions and checking for a "jump" of composition profiles at bifurcation points. The main result is a systematic and exact method of selecting appropriate pairs of boundaries for a given type of column, which demarcate distillation regions. The distillation regions overlap. The common area of two distillation regions is parametrically sensitive, and it determines the possibilities of crossing (at a finite reflux) the distillation boundaries defined for a total reflux or reboil ratio. However, such columns may be difficult to start up and control. An algorithm determining the distillation regions, which takes into account properties of distillation boundaries, has been formulated. 0 2005 American Institute of Chemical Engineers AIChE J, 52: 532-544, 2006