The chemically modified surface plays an Important role In retention In reversed-phase liquid chromatography. One surface factor that might affect retention Is solute expulsion due to partial ordering of the grafted chains at sufficiently high grafting densities. This study has shown by the use of the electron paramagnetic resonance technique of spin probing that spectroscopic evidence exists for an Increase In molecular order of the bonded octadecyl phase as a function of grafting density. The spin probe, DOXYL-cholestane, shows significant Increase In the order parameter, S, as a function of Increased surface coverage. In addition, DOXYL-cholestane undergoes a decrease In rotational freedom and an Increase rotational anisotropy as the grafting density Increases.