The Reddy-O'Shea (RO) and Cuadros-Mulero (CM) equations of state for two-dimensional Lennard-Jones fluids have been used to obtain the isosteric heat of adsorption of simple fluids onto flat surfaces. The results are compared with a set of available experimental data, observing a large degree of agreement, especially when the more simple CM equation of state is used. Another advantage of the CM equation is that it is based on the Weeks-Chandler-Andersen theory for simple fluids. In light of these results, we propose, as a first approach, the application of the CM model to describe the thermodynamics of the adsorption of simple fluids onto flat surfaces.