Two different types of silica substrates uniformly modified with reversed-phase high-performance liquid chromatographic (RP-HPLC) phases obtained with mono-, di- and trifunctional octadecylsilanes were subjected to artificial ageing under simulated routine conditions and were subsequently analysed and evaluated. Changes in the properties of the bonded phase packings were characterized and quantified with chromatographic techniques, solid-state NMR and elemental analysis. These changes were correlated with selectivity, loss of silanes, gain in silanol content and rearrangement of the silica to silane bonding. When eluents of extremely high pH were used, the multifunctional octadecylsilane stationary phases showed a higher resistance towards ligand stripping. The substrate shielding of difunctional octadecylsilanes showed superior characteristics due to direct surface attachments. The rigidity of the silica substrate used for modification influences the stability of the RP-HPLC phases, especially for the monofunctionally modified octadecylsilane stationary phase. With the use of multifunctional C18 silanes for modification, the influence of the characteristics of the substrate on the stability of the RP-HPLC phases was reduced compared with monofunctional C18. In contrast to monofunctionally modified phases, the polar selectivity of the multifunctionally modified phases remained more or less constant after intensive use with aggressive eluents. A chromatographic characterization method for obtaining information about the stability, capacity, selectivity and silica degradation of different RP-HPLC stationary phases is also presented.