Liquid-liquid extractions in microporous hollow fiber modules are unusually fast because of the large surface area per volume in these modules. Extractions of p-nitrophenol into amyl acetate show no membrane resistance, and hence are especially rapid. Extractions of acetic acid into methyl amyl ketone are controlled by the membrane resistance. The results show that extractions in hollow fibers will be fastest when the fibers are wet by the fluid in which the solute is more soluble. They also show when fiber modules are a sound inexpensive alternative to centrifugal extractors.