The intraparticle obstructive factors of several endcapped C-18-bonded silica particles with different C-18 surface densities (0, 0.42, 1.01, 2.03, and 3.15 mu mol/m(2)) were measured. A column packed with the reference neat silica was also used. The stop and flow experiment technique was applied, with the injection of a pulse of 5 mu L of a dilute solution of thiourea, a nearly unretained compound. The contribution of radial diffusion was also determined by injecting a semi-infinite concentration plug into the column. Experimental results demonstrate that the stop and flow method provides accurate and precise values of the apparent diffusion coefficient D-app of the sample inside the column. Radial diffusion plays a negligible role compared to that of axial diffusion. A theoretical model of diffusion through the packed bed is proposed, based on the sum of parallel diffusion fluxes in the solutions contained in the external and the internal porosities of the bed, and onto the surface of the solid adsorbent. The agreement between the experimental values of the diffusion coefficient D-app and the values predicted by the model demonstrates that: (1) surface diffusion plays a major role in axial dispersion (30%), despite the small retention and isosteric heat of adsorption Q(st) of thiourea, and (2) the intraparticle obstructive factor, gamma(i), decreases significantly, from 0.82 to 0.56 from the neat silica particles to the C-18-bonded silica particles with a 3.15 mu mol/m(2) surface density. (c) 2006 Elsevier Ltd. All rights reserved.