The deactivation and coating of 50-µm I.D. capillary columns was studied, using borosilicate and fused-silica columns with over 105 theoretical plates. A device is described for the convenient introduction of fluids by means of nitrogen pressure up to 80 bar. The device permits fluid switching while the working pressure is maintained. Established procedures for leaching and polysiloxane degradation, which are normally used with 0.25-mm I.D. capillaries gave low coating efficiencies when applied to the 50-µm I.D. columns studied. Modified procedures are described. For borosilicate columns an acceptable level of deactivation has been achieved. A slight residual activity towards alcohols and amines could not be prevented. Excellent deactivation was observed for the fused-silica columns, even when only 15 pg of polar test compounds where injected. Chromatograms demonstrating the separation of terpenes and of underivatized drugs are given.