The film of stationary phase on the wall of a capillary column and that of the phase solution during both static and dynamic coating is subject to Rayleigh instability, which is quite independent of so-called wettability. A theory is developed which shows that the logarithmic growth rate of Rayleigh instabilities is proportional to thesurface tension and to the third power of the film thickness, and inversely to the viscosity and to the fourth power of the capillary diameter. Determination of the variation of the viscosities of stationary phase solutions with concentration in coating solvents, and the variation of the viscosities of neat stationary phases with temperature, both revealed that heating and/or diluting changed the viscosities of phases with n-electron-containing groups much more than for polydimethylsiloxanes. Rayleigh instability is therefore more important during coating of phenyl-containing phases such as OV-17, and later during column operation. The efficiencies of capillary columns of different diameters coated with a number of phases under different conditions of temperature and coating rate, and then operated at different temperatures were in good agreement with the predictions of the theory.