Distillation in hollow fibers can give better, more productive separations than distillation in random or structured packing. Since the hollow fibers used have little resistance to mass transfer, the selectivity of the separation depends on the relative volatility, just as in conventional distillation. Because the fibers are nonporous, the distillation can operate at high flows which normally cause flooding, and at low flows which normally compromise loading. As a result, the turndown ratio is nearly infinite and the height of a transfer unit can be as small as desired. Experiments do give values consistent with correlations developed for other membrane modules, although the practical value of hollow fiber distillation is uncertain because of the materials limitations.