Spherosil, a spherical porous silica, has been investigated for use in high speed liquid-solid chromatography and compared to Lichrosorb Si-60, using three phenothiazines as test solutes. Particle size distribution for four different size ranges with nominal mean diameters of 5, 10, 20 and 40 µ m is given. Distribution is very homogeneous. Columns from 15 to 100 cm in length and 1/4 or 1/8 in.o.d. have been prepared and efficiency measured by determination of HETP with the three phenothiazines: [methylamino-3-propyl]-10-chloro-3- phenothiazine [k' = 3]; [N-methyl-N-[dimethylamino-3-propyl]amino-3-propyl]-10-chloro-3-phenothiazine [k' = 6.5]; and [dimethylamino-3-propyl]-10-chloro-3-phenothiazine-N-oxide[ k' = 15]. Influence of column length and incidence of bead diameter have been studied. 1/4 in.o.d. columns are easier to fill than 1/8 in.o.d. columns for Spherosil 5 µm and have, therefore, a greater efficiency. HETP, H, varies according to flow rate as H = DVn with 0.4 < n < 0.6 and according to particle size as H = A dβp with 1.7 < 0 < 1.8. The best figures for H are between 0.1 or 0.2 mm for a flow rate of 1800 ml hr−1 cm−2 [ k' = 15]. The separation of a mixture of 6 phenothiazines with the mobile phase, anhydrous ethyl acetate 60 V, water saturated ethyl acetate 20 V, anhydrous methanol 20 V, 33%.aqueous solution of ethylamine 0.25 V, is given. Its duration is 20 min. instead of 90 min. by thin-layer chromatography.