The statistical rate theory predicts that the observed kinetics of gas adsorption on solid surfaces may be affected by the construction of an experimental setup, and by the conditions at which the kinetic experiment is carried out. The current paper is a further extension of our recent theoretical studies of the kinetics of adsorption on/from energetically heterogeneous surfaces for the conditions when, at any instant of time, the investigated system is far from equilibrium (Rudzinski, W.; Panczyk, T. J. Phys. Chem. B 2000, 104, 9149). It is shown that under certain experimental conditions, concave kinetic isotherms might be observed in some adsorption systems. As such data are rarely reported in the literature, the conclusion is that the majority of the reported kinetic experiments were carried out under the conditions at which the adsorption systems could be considered as close to equilibrium. This is because for such experimental conditions the theory predicts existence of only convex kinetic isotherms for both energetically homogeneous and heterogeneous solid surfaces. For all these experiments an easy way is shown how to deduce the form of the kinetic isotherms from the equilibrium adsorption isotherms.