The influence is considered of mass transfer oil distillation column design. The column design equations incorporating mass-transfer effects are similar to the conventional design equations and are as easy to solve numerically. Mass-transfer effects do not influence pinch-point curves and pitchfork distillation boundaries. Mass transfer does, however, change the composition trajectories, which might thus cross the pinch-point curves. Inclusion of mass transfer in the design equations allows its to estimate the actual number of stages (or the column height). The curvature of composition trajectories may have a significant impact on the total number of stages and oil the feed stage location. Mass-transfer effects do not influence the minimum reflux,for columns involving a very sharp split. However, and contrary to previous reports, mass-transfer effects may strongly effects the minimum reflux for less than very sharp separations. Mass-transfer effects should be taken into consideration when trace components are a concern. A feasible design based on residue curve maps might be rejected by a design method based oil mass transfer. More important, however, the converse also is trite. (C) 2005 American Institute of Chemical Engineers.