The Influence of the llgand alkyl chain length, chemically bonded at the surface of a silica substrate, on statlonary phase stabilty In llquld chromatography practlce is reviewed. Several factors affecting long-term stability of modern reversed- phase high-performance liquid chromatography (RPHPLC) phases are consldered and their lndlvldual contrlbutlons are evaluated In this paper and the following paper In this issue. The stationary phases under study were identically modifled on the same batch of sillca substrate to ellmlnate differences In substrate properties and synthesis conditions. Modiflcatlons with llgand alkyl chain length between C1 and C18 were performed such that an approximately equal ligand density was obtained for the the seven RP-HPLC phases studied. These n-alkyldimethylsiloxysilane bonded phases were exposed to sknulated aglng experlments. A subsequent chromatographic characterizatlon regarding changes In capacity, lipophilic and polar selectlvlty, and sillca degradatlon related separation performance was carried out. Comparison with results determined by other characterization methods, llke bulk analysis, elemental analysls, and solid-date 29Si crosspolarizatlon magic angle spinning NMR revealed that with longer n-alkyl llgands gradually better substrate shieldlng properies were obtained. The ligand alkyl chain length affects the stability of reversed phases to a large extent. Short ligand modified phases changed drastically uslng relatively aggresslve eluents. Organlc modifier rlch eluents combined with longer ligand modifled phases are preferable for a longer llfetlme of RP-HPLC columns, especially when aggressive eluents are used.