The use of column packing with particle sizes of 10 μ in different modes of column chromatography, liquid—solid (LSC), bonded phase (BPC) and gel permeation (GPC), was investigated. LSC columns of very high efficiency, 10 effective plates/sec. and high reproducibility were produced by packing μPorasil, a silica gel with a surface area of 400 m 2/g and an average particle size of μ, into drilled stainless-steel tubes. Bonded phases were prepared by the reactionof monomeric organosilanes with μPorasil to form monomolecular bonded layers. Octadecyl, aminopropyl and cyanopropyl groups were bound to the surface. Packed columns of the monomolecular, bonded phases gave very high efficiencies. More than 300 theoretical plates and 10 effective plates/sec can easily be obtained on a single 30-cm-long column at a carrier velocity of 0.7 cm/sec. High-speed GPC was performed by using packed columns of μStyragel 2, a porous polymer of styrene and divinylbenzene, with average particle sizes of 20 μ. Columns packed with μStyragel ( dp ??? 10μ) gave 4000–10,000 theoretical plates/30 cm of column, depending upon the carrier velocity. Molecular-weight distributions of many polymers can be obatined in 10–15 min. In addition, the columns were also run at lower carrier velocities to separate small molecules which differ by as little as one methylene group.