The diffusional properties of molecules in solution vary dramatically upon addition of a solid chromatographic phase. This effect can be monitored via pulsed-gradient spin - echo NMR used in conjunction with moderately fast rotation of the sample (high-resolution magic angle spin- ning) to produce exploitable spectra. The molecular diffusion coefficients observed in this condition are aver- ages reflecting the equilibrium population distribution among the different phases. It is thus possible to use this information for investigating a crucial step of reversed- phase chromatography, namely, the partitioning of the analyte between different phases. In this work, we de- scribe the evolution of the apparent diffusion coefficient of typical solutes for water/acetonitrile solvent mixtures of varying proportions.