Abstract Silane-coupling agents (SCA) were applied for deactivation and increasing the wettability in both glass and fused-silica capillary columns prior to coating with medium-polarity stationary phases, such as Carbowax 20M or Superox. γ-Glycidoxy-propyltrimethoxysilane induced a satisfactory surface deactivation, offering the possibility of preparing highly efficient columns in a one-stage (fused-silica) or two-stage (glass) process. Further laboratory experiments indicated the potential of SCA for generating the effect of phase immobilization via a radical-induced grafting and cross-linking mechanism. Immobilized-phase (IP) Carbowax 20M capillaries, operating in the range 80–280°C, started to destabilize only above 305°C. A gas chromatographic (GC) evaluation of the IP columns before and after methanol flushing demonstrated different polarity behaviour. The IP columns are suitable for GC analyses of broad-range mixtures [C 6C 30] of fatty acid methyl esters including cis-trans isomers.