The most popular formula used in the literature about liquid/solid adsorption kinetics to describe diffusion-controlled processes is the intraparticle diffusion (IPD) equation. However, this formula was introduced originally for pure diffusion. It does not account explicitly for the effect of adsorption (except in the limit of very low adsorbate concentration). In this work, the problem of diffusion-controlled kinetics is studied by using a diffusion-adsorption model which should hold when the solute concentration in the external solution is sufficiently high. The case of a finite amount of solute initially in the stirred batch adsorber is solved analytically. For short times, the formula for the uptake turns out to have the same form vs. time as the IPD equation. However, it also predicts a decrease of the fractional uptake with the initial bulk concentration, as observed in the literature, and it shows that the IPD diffusion coefficient is a lumped parameter depending on the experimental conditions. These theoretical results are used for a discussion of the IPD equation and for descriptions of experimental results taken from the literature.