This study shows that adsorption experiments under flow allow determination of a phenomenological kinetic constant k(a), provided the system corresponds to an appropriate regime. Measurements of the adsorption rate along a capillary enable calculation of both the kinetic constant k(a) and the diffusion coefficient D of the solute, and the latter can be compared with independent results from other techniques. Alternatively, measurements at a single position give an estimate of k(a) when the diffusion coefficient D is known. The conditions under which these results may be derived are discussed in detail.