Liquid-solid chromatographic columns were packed by a high-pressure, balanced-density slurry technique. Effect of particle diameter (dp) on column performance and permeability was investigated for six silica gels of 5 ⩽dp ⩽40 µm. Column efficiency, represented as HETP, was dependent on dp1.s for an azo dye test solute. HETP values for columns of 5 µm silica were found tobe two orders of magnitude better than 5 µm silica dry-packed by conventional techniques. The “effective” particle diameter was best characterized by calculation from electron micrographs. Column permeability was found to be proportional to dp1.s. Performance factors and effective plates per second systematically increased with decreasing dp and indicated even greater increases in column performance may be expected for yet smaller particles. Columns of porous silica of dp ⩽ 20 µm demonstrated significantly better performance than Corasil II, a porous layer bead. The dependence of plate height on length from 15 to 100 cm for 2.4 mm, i.d., columns packed with 10 µm particles was investigated. Little change in efficiency was observed up to lengths of 50 cm but columns of 70 cm and 100 cm did not give the expected plate number. However, two 50 cm columns in series gave only a slight decrease in plates.