The kinetics of adsorption processes in solution onto adsorbents having different geometric shapes have been explored theoretically. The basic assumptions were (i) the diffusion of adsorbate in quiescent homogeneous solution with no convection, (ii) simple Langmuir-type adsorption kinetics and isotherm, (iii) the bulk concentration of the adsorbate in solution being sufficiently high to stay constant during the adsorption process, and (iv) the geometric shape of the adsorption surface being planar, spherical, or cylindrical. The diffusion equation with the time-dependent boundary conditions implied by the adsorption process was shown to lead to a nonlinear Volterra-type integral equation which is common for the three adsorbent geometries with a single definitive parameter, geometric factor, varying between 0 and 1. A numerical method was developed far solving this equation, and approximate analytical solutions were derived for the very beginning and the very end of the adsorption process. Implications of the results for the analytical methods based on the use of microparticles, such as various immunoassays, are discussed.