The initial adsorption kinetics in stagnant flow of both rigid spherical poly(propylene imine) dendrimers as well as of flexible branched poly(ethylene imine) with similar building-blocks on glass are quantitatively described as Smoluchowski-Levich convective-diffusion-controlled up to concentrations C-b approximate to 10 mg/L using a sticking probability beta, defined as the ratio between the experimental adsorption rate and the theoretical prediction. This parameter beta accounts for the nonideality of the substrate: There is only a limited amount of surface sites in the glass get-layer available for adsorption, and this affects the collision efficiency. beta is on the order of 0.03 for the small spheres and 0.8 for the flexible polyelectrolyte, perfectly in line with the expectation that the flexible polyelectrolyte, able to conform along the surface, has a much higher chance of finding a site within its contact area than the small dendrimers. At higher concentrations, the experimental data no longer show the predicted linear concentration dependence. Whereas for parallel plate flow studies in the literature the breakdown can be related to the neglect of transient concentration effects in mass transport using the Smoluchowski-Levich approach, for stagnant flow, such a simple relation does not exist. It is calculated for the latter setup that the time scales needed for the concentration to settle throughout the cell would be longer for the larger poly(ethylene imine) than for the dendrimers. This is in contrast with the experimentally observed trend: time scales over which deviations between the Smoluchowski-Levich approximation and the data occur are longer for the faster diffusing dendrimers than for PEI, and the breakdown can thus at least not directly be related to transient concentration terms. It is suggested that the large difference in the sticking probability of these different types of materials on glass has to be taken into account in a complete numerical analysis of the mass transport equation, especially at short time scales.