A quantitative theory of kinetic-diffusive-convective adsorption in a flow cell for polyelectrolyte and mixtures of ions in solutions onto planar surfaces in the framework of the Nernst-Planck approach has been developed by using the new approaches : (a) The effect of the convective transport is estimated in the analytical form. The boundary condition for the rate of adsorption at the interface (y = 0) is derived for the kinetic-diffusive-convective-controlled adsorption in a flow cell, taking into account both the normal and the axial fluxes in the flow cell. (b) The total electrical potential in the double layer, phi(x, y, t), does not equal the electrical potential, phi(rho)(x, y, t), due to the total electrical charge, rho(x, y, t), in the double layer. It is shown that the total electrical potential in the double layer must be found only from the integral form of the charge-balance equation in the double layer. Equations were derived to calculate the rate constant of the adsorption process in a flow cell for short and long times and the time needed to attain the equilibrium state for the adsorption processes. It is shown that the adsorption kinetics of polyelectrolyte over a wide range of times is governed by : (I) the adsorption kinetics of polyelectrolyte on a planar surface of the bare interface for short times, (II) the adsorption kinetics at the interface as well as simultaneous diffusion in the adsorbed and double layers for intermediate times, and (III) the adsorption kinetics at the interface for long times.